Never. Or almost never. Here’s how I suggest handling search and perception:
- When PCs pass through an area, describe everything they could possibly see.
- When PCs perform specific actions, like looking behind a tapestry, add to the description.
- When PCs actually search an area, describe everything they can find in that area.
If there is a mechanism attached to a container, say “that container has some kind of mechanism that looks like it would be triggered by opening it” and stop. Do not assume the PCs automatically open trapped containers, turn knobs, pull levers, drink from pools of water, or anything else. Let the players decide what to do.
There are only a few so-called “perception checks” I would ever make.
Search for secret doors. A secret door is designed to blend in, so it’s an exception to the “describe everything that could be seen” rule. The “find secret doors” roll determines whether PCs suspect something is there or not. Triggering the mechanism that opens the secret door will reveal it, whether the roll is successful or not.
Hear noise. All obvious sounds, even low ones, will be heard automatically, with no roll required. In cases where there is a monster or mechanism that is normally silent, but that might make a sound, or makes a sound that is normally imperceptible, PCs (especially thieves) have a chance to hear the noise. I would use this roll for a monster breathing quietly behind a heavy door, for example, but would not use it for a murmured conversation behind a door, which would be detected automatically without a roll, as long as the PCs listen at the door.
Knowledge check. In some cases, characters with a specific background will be able to find things easier, or will have extra information. For example, a dwarf can automatically detect a stone wall that is able to shift position, a thief will recognize if a lock is trapped, an alchemist may recognize a chemical residue on the floor. These will usually not require a roll. However, characters who do not have an appropriate background may still have a chance to recognize the same information if their Intelligence is high. I would use 5+ on 1d6 for PCs with Int 16+ in such situations.
Bad conditions. Blurred vision or a smoke-filled room, for example, may make otherwise obvious things require a roll to notice. Running normally makes things like tripwires impossible to see, but you could allow PCs with Int 16+ a chance to notice them moments before triggering them.
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