The following is the text from the Empty Room, Tricks, and Traps document. The additional information is added to increase agency.
Pressure Plates: These may have a variety of resistances, each pressure plate requiring different weights to trigger it, or they may even keep a tally of the amount of weight that has crossed them. Note that prodding the floor with a 10’ pole will be less and less likely to discover pressure plates the larger and more weight that is required to trigger the plate. Pressure plates may be any size, from that of a small dime to the size of a large room. They are often the width of a corridor or on the threshold of a door. They may also be triggered, not when stepped on but when the weight leaves the plate or the plate may even disable the trap.
Pressure plates cannot be completely integrated with floors. There is always a crevice between the pressure plate and the rest of the floor. These crevices can be covered in plaster but this measure only lasts until the floor is triggered. Once triggered, the cracked plaster could provide a clue to the location of the pressure plate. Water when poured on a floor will travel down these crevices. It is possible to cover a pressure plate with a permanent illusion. The magic aura is detectable via magic and by thieves that can detect traps.
Pressure plates do not consistently trigger. There is only a percentage chance, usually 50%, that it will trigger as each person crosses it.
Because the design is a plate that rests over something depressable, the plate is often detectable by the fact that it is unsteady or wobbles.
Pressure plates are often covered by carvings or integrated with mosaics designed to hide them from detection.
Whatever they trigger will often leave marks, discoloration, or other indicators.
Latch or Switch: These refer to any device that triggers a trap based on the stated action of the player character. (e.g. “I open the chest,” or “I sit on the bench”). It often consists of a literal latch or switch. A latch is detectable by looking for where it is attached or where it connects to the trap. A switch is detectable by some give or looseness in the object to the point where the switch is triggered. This type of trigger also includes gear and lever triggers, though the players should be unaware they are triggering a gear or lever.These are often located in doors and hinges. However more complex latches and switches are attached to objects, such as books, chairs, and torch sconces.
There are several methods to detect these. The object is usually a false object, detectable due to discoloration or structure. Sometimes real objects are used (such as a real book in a bookcase) but constant use eventually causes damage. The objects are often hid among several actual real items to increase the illusion.
Because the objects don't usually move (except to trigger the switch). They may be covered in dust, or show differing signs of wear compared to other similar objects. If the object is freestanding, it often must be attached to a surface with a hinge or nails. The latch is often triggered by cables or ropes hidden within the walls.
If the object moves against another surface, such as a torch sconce that rotates against the walls, it will leave marks from the use.
Cheap latches will have visible hinges, making it very obvious.
In a high quality latch or switch with a well made object, the presence of something unusual can be detected by noticing that the area where the cable runs is hollow upon a close inspection.