These “mechanical” triggers are very simple—any object that covers a pit that doesn’t look like the a pit is a lid. This also refers to false coverings that open into a trap. Prodding and tapping are very effective at detecting these kinds of traps. Pits, like pressure plates, usually have a percentage chance to not be triggered. Since lids are literal covers for holes, water usually tends to be very effective in detecting these traps.
There are different types of pits lids: open, breakaway, latch, teeter-totter, and illusion.
Breakaway pits are camouflaged, but any testing or prodding will indicate that the floor is not solid, or that the covering has some give to it. Consider the terrain and the material used to cover the breakaway pit. Leaves and sticks outside, a carpet indoors.
Latch pits have moving parts. There is some support for the latch, and usually some way for the latch to reset. This means a wheel or gear hidden nearby. Also, it is more difficult to hide the seam of a latched pit.
Teeter-totter pits are easy enough to detect by prodding, but as a side effect from heavy use, may not center correctly. One side may be raised up an inch or more, while the other is low. Or if it is poorly made, the wrong side of the lid could be visible.
Illusionary pits create a very realistic looking floor, making detection by vision almost impossible. The illusion isn’t solid and doesn’t interact with the environment. Detect magic makes it obvious, but fog rolling along the ground will drop right through. It doesn’t block sound or airflow, all of which can provide clues to the illusion.
B2: Keep on the Borderlands by Gary Gygax
A. KOBOLD LAIR: There is a 2 in 6 chance that as the group enters the cave-like tunnel, 8 kobolds will come out from hiding in the trees above and attack. . .Each carries d8 silver pieces.
Note: 30´ inside the entrance is a pit (B). There is a 3 in 6 chance that each person in the front rank will fall in unless they are probing ahead. There is a 1 in 6 chance that individuals in the second rank will also fall in, but only if they are close to the first rank and the character ahead has fallen in. The pit is 10´ deep, and those falling in will take 1-6 points of damage. The pit lid will close, and persons within cannot escape without aid from the outside. The noise will attract creatures from areas 1. and 2. Planks for crossing the pit are stored at #1., beyond.