Ok. First with the skill text. If you're interested in the use of these skills in Phandelver or the analysis, skip down past the skill text. It is provided first as reference.
Perception. Wisdom (Perception) check lets you spot, hear, or otherwise detect the presence of something. It measures your general awareness of your surroundings and the keenness of your senses. For example, you might try to hear a conversation through a closed door, eavesdrop under an open window, or hear monsters moving stealthily in the forest. Or you might try to spot things that are obscured or easy to miss, whether they are orcs lying in ambush on a road, thugs hiding in the shadows of an alley, or candlelight under a closed secret door.
Investigation. When you look around for clues and make deductions based on those clues, you make an Intelligence (Investigation) check. You might deduce the location of a hidden object, discern from the appearance of a wound what kind of weapon dealt it, or determine the weakest point in a tunnel that could cause it to collapse. Poring through ancient scrolls in search of a hidden fragment of knowledge might also call for an Intelligence (Investigation) check.
And special rule text:
Hiding. When you try to hide, make a Dexterity (Stealth) check. Until you are discovered or you stop hiding, that check's total is constested by the Wisdom (Perception) check of any creature that actively searches for signs of your presence.
You can't hide from a creature that can see you, and if you make noise (such as shouting a warning or knocking over a vase), you give away your position. An invisible creature can't be seen, so it can always try to hide. Signs of its passage might still be noticed, however, and it still has to stay quiet.
In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you. However, under certain circumstances, the Dungeon Master might allow you to stay hidden as you approach a creature that is distracted, allowing you to gain advantage on an attack before you are seen.
Passive Perception. When you hide, there's a chance someone will notice you even if they aren't searching. To determine whether such a creature notices you the DM compares your Dexterity (Stealth) check with that creature's passive Wisdom (Perception) score, which equals 10 + the creature's Wisdom Modifier, as well as any other bonuses or penalties. If that creature has advantage, add 5. For disadvantage, subtract 5.
For example, if a 1st level character (with a proficiency bonus of +2) has a Wisdom of 15 (a +2 modifier) and proficiency in Perception, he or she has a passive Wisdom (Perception) of 14.
What Can You See? One of the main factors in determining whether you can find a hidden creature or object is how well you can see in an area which might be lightly or heavily obscured, as explained in Chapter 8.
Surprise. . . If neither side tries to be stealthy, they automatically notice each other. Otherwise the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side. Any character or monster that doesn't notice a threat is surprised at the start of the encounter.
If you're surprised, you can't move or take an action on your first turn of the combat and you can't take a reaction until that turn ends. A member of a group can be surprised even if the other members aren't.
Vision and Light. . . A given area might be lightly or heavily obscured. In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.
A heavily obscured area--such as darkness, opaque fog, or dense foliage--blocks vision entirely. A creature in a heavily obscured area effectively suffers from the blinded condition. . .
Finding a Hidden Object. When your character searches for a hidden object such as a secret door or trap, the DM typically asks you to make a Wisdom (Perception) check. Such a check can be used to find hidden details or other information and clues that you might otherwise overlook.
In most cases, you need to describe where you are looking in order for the DM to determine your chance of success. For example, a key is hidden beneath a set of folded clothes in the top drawer of a bureau. If you tell the DM that you pace around the room, looking at the walls and furniture for clues, you have no chance of finding the key, regardless of your Wisdom (Perception) check result. You would have to specify that you were opening the drawers or searching the bureau in order to have any chance of success.
Massive, Endless Spoilers. Seriously, this article is about perception/investigation. Every hidden secret and surprise monster is listed below:
"Make a Dexterity (Stealth) check for the goblins, rolling once for all of them. Roll a d20, add the goblins' Stealth skill modifier (+6) to the roll, and compare the result to the character's passive Wisdom (Perception) scores. Any character who's score is lower than the goblins' check result is surprised and loses his or her turn during the first round of combat."
Finding the Goblin Trail
". . . A character who succeeds on a DC 10 Wisdom (Survival) check recognizes that about a dozen goblins have come and gone along the trail, as well as signs of two human-sized bodies being hauled away from the ambush site. . ."
"If the characters are searching for traps, the character in the lead spots the trap automatically if his or her passive Wisdom (Perception) score is 12 or higher. Otherwise, the character must succeed on a DC 12 Wisdom (Perception) check to notice the trap."
"The character in the lead sspots the hidden pit automatically if his or her passive Wisdom (Perception) score is 15 or higher. Otherwise the character must succeed on a DC 15 Wisdom (Perception) to spot the hidden pit."
"Sound. The sound of water in the cave muffles noises to any creatures that aren't listening carefully. Creatures can make a DC 15 Wisdom (Perception) check to attempt to hear activity in nearby chambers."
Cragmaw Goblin Lookouts
"Characters moving carefully or scouting ahead might be able to surprise the goblin lookouts. Have each character who moves ahead make a Dexterity (Stealth) check contested by the goblins' passive Wisdom (Perception) score to avoid being surprised. . ."
Goblin on bridge
"Any character who can see the bridge in area 5 might also notice the goblin guarding the bridge. Doing so requires a Wisdom (Perception) check contested by the goblin's Dexterity (Stealth) check result.
The goblin notices the characters if they carry any light or don't use stealth as they approach the bridge. The goblin does not attack. Instead, it attempts to sneak away to the east to inform its companions in area 7 to release a flood. . . The goblin moves undetected if its Dexterity (Stealth) check exceeds the passive Wisdom (Perception) score of any character who might notice its movements."
Aside: Later it's noted that this goblin is "lazy and inattentive".
"Secret doors are made of stone and blend in with the surrounding walls. Spotting a secret door from a distance of no more than 10 feet without actively searching for it requires a passive Wisdom (Perception) score of 15 or higher, whereas a character who takes the time to search the wall can find the secret door with a successful DC 10 Wisdom (Perception) check."
Satchel Hidden in Water
"A waterproof satchel hangs from a submerged rope attached along the south wall of the cistern, about 2 feet below the surface of the water, but can be found with a successful DC 15 Wisdom (Perception) check or automatically by a character probing the cistern with a pole or jumping in."
Pit trap in hall
"A character searching the hall for traps can spot the covered pit with a successful DC 15 Wisdom (Perception) check. A successful check also reveals narrow ledges on the north and south sides of the pit."
"A character next to the bridge can discern that the construction is faulty with a successful DC 15 Intelligence (Investigation) check."
Listening at the door
"A character who listens at this door with a successful DC 10 Wisdom (Perception) check hears several gruff voices issuing demeaning commands in the goblin tongue."
"A character who listes at the door with a successful DC 10 Wisdom (Perception) check hears the villains within engaged in a game of knucklebones"
"Faint bubbling and dripping sounds can be heard though either door of this room with a successful DC 15 Wisdom (Perception) check"
At this point, that's enough examples of using Wisdom (Perception) as a listen check. In the future, I'll only note exceptional cases.
"Two twig blights hide among the weeds that flank the cottage's open doorway. Make a Dexterity (Stealth) check for the blights, and compare the result to the passive Wisdom (Perception) scores of the characters to determine if the blights are spotted"
"The overgrowth conceals a deadly threat--six twig blights lurking among the ordinary foliage. Spotting them requires a successful Wisdom (Perception) check challenged by the blights' Dexterity (Stealth) check"
"A successful DC 10 Wisdom (Perception) check reveals the old chest among the roots of the tree growing up through the house."
"Six twig blights lurk in the thicket south of this ruin. Allow each character to attempt a Wisdom (Perception) check contested by the blights' Dexterity (Stealth) check to avoid being surprised by them."
"The party can attempt one DC 15 Wisdom (Perception) or DC 10 Wisdom (Survival) check per hour to find the camp."
"Spotting the tripwire requires a passive Wisdom (Perception) score of at least 20, or a successful DC 10 Wisdom (Perception) check if the characters are actively searching for traps in the area."
"Compare the grick's Dexterity (Stealth) check to the character's Wisdom (Perception) checks (or their passive scores) to determine who among them is surprised."
"A character who succeeds on a DC 15 Wisdom (Perception) check spots a footpath leading up to the hidden entrance. If the characters are actively searching the outside of the castle for a hidden entrance, they can make a DC 10 Wisdom (Perception) check to spot the canvas 'door'."
"All that remains of the tower's second floor is a jagged ledge, upon which sits a battered wooden chest. The chest is hard to see from the floor, requiring a successful DC 15 Wisdom (Perception) check to notice."
Wave Echo Cave
"Any character who isn't watching the ceiling is surprised unless his or her passive Wisdom (Perception) score is higher than the stirge's Dexterity (Stealth) check total. . ."
Wizard's Quarter's Treasure
"A handful of magically preserved tomes remain the shelves. . . but one has a map sewn into its cover. The map's presence can be discerned with a successful DC 12 Intelligence (Investigation) check."
Collapsed Cavern Treasure
"Buried under heavy rubble at the bottom of the rift is the crushed skeleton of a dwarf wearing gauntlets of ogre power. The remains are hidden from view but can be found with a successful DC 20 Wisdom (Perception) check. Each character searching can attempt one check per hour."
Temple of Dumathoin
"However, the jewels are clever fakes made of worthless glass, as a close inspection and a successful DC 15 Intelligence (Investigation) check reveals."
Perception seems to be heavily leveraged to take the place of Spot/Listen, serving the role of a surprise roll in classic D&D. It also seems to serve double duty as a search skill sometimes. Though when Wisdom (Perception) is appropriate and when Intelligence (Investigation) is appropriate is not clear from the text or examples. There also appears to be some overlap with perception as a skill versus survival in the wilderness.
For example: The Satchel requires a Perception check to find, as well as the chest hidden among the tree roots, whereas the wizard's quarter treasure requires an Investigation check.
I am assuming that the original advice, that if the characters look where a thing is, that a roll isn't necessary, such as the second floor battered ledge in the owlbear tower or if a character explicitly checks out the roots from the Thundertree ruins treasure.
Why doesn't the collapsed cavern treasure require an Investigation check? You can't see the corpse and it takes an hour to look. It seems like Investigation ("deduce the location of a hidden object") appears to be the appropriate check to find the corpse with the gauntlets. I am almost certain that this is an error.
There are a lot of strange inconsistencies however. Traps are checked versus passive perception if not searching, and versus active perception if checking with the same DC, however some checks appear to have different difficulties in different cases. There's an active check outside the Cragmaw Castle Ruined Tower at DC 15 that drops to 10 if you're "actively searching. . . for a hidden entrance". This happens again with the trap which has a DC 20 passive check if you're not looking or a DC 10 check if you are.
I tend to assume that the characters are competent. That when they move they are always searching for enemies and that they are moving along taking precautions against hallway traps. Some of the examples in the text seem to indicate that every time the characters move they have to declare they are searching or they only receive their passive perception score (and sometimes not even that at all). I don't know if I'm interested in requiring that kind of hoop-jumping--especially in a heroic adventure game. Generally in my B/X game I only require players to state that they search chests and doors for traps.
Conclusion or How I'm Going to Run ItWisdom (Perception) is used for surprise.
- Wilderness Travel
- During wilderness travel when travelling at a normal pace, it's a passive check unless specifically scouting ahead.
- At a slower than normal pace it's an active roll.
- if moving faster than normal the check is at disadvantage (Passive - 5).
- During camp it's a passive check unless a watch is set.
- The character that's on watch can make an active check.
- If you're sleeping, your check is your passive check at disadvantage.
- In a dungeon or interior it's assumed that you're looking for opponents or traps actively. Checks are always made actively once the threshold to adventure is crossed.
Wisdom (Perception) is used for noticing and discovering
- No check is necessary if the player asks to look in the appropriate place
- Doors and hidden objects are checked versus the passive value.
- If they want to bypass play to search an area and the area contains a hidden item, the DM can roll a perception check for the player searching. This takes time and possibly draws wandering monsters.
Intelligence (Investigation) is used for searching
- There is no passive score. If you have to search a library for a book, search rubble for a treasure, discover the meaning of some event or item, toss a house for an item, or a find a passage in a book, this is the skill to use.
- This also gives you information about your environment that is A) optional and B) not straightforward.
All the text regarding those skills and their use in the starter set is above for your perusal.