In a room I could pretty easily give them some sort of clue that the wall was a bit funny, but at just a turn in an otherwise featureless corridor? I'm struggling a bit. Any ideas on how I can indicate the something is awry, without just putting up a "Secret Door Here" sign?
Some other ideas not mentioned yet:
- stone or other surface could be somewhat discolored or the secret door doesn't quite match the wall surface (so you can do this with color, or texture: perhaps the secret door area is smoother, or rougher, or slightly oily, or whatever)
- the environment around the secret door could be cleaner: not necessarily the door itself, but the floor/wall area, due to occasional usage (which would mean the floor/walking area is clearer [or that a gelatinous cube had recently passed through

], or that there aren't any cobwebs handing from the ceiling, etc.
- the trigger mechanism could have all of these features, but be not in immediate sight of the secret door: so, the PCs find the trigger, and trigger it, but don't see the secret door immediately but then start to look around in the vicinity to see where it is, of course; it could be on the other side of a wall, or it could be a multi-part trigger (two triggering mechanisms to open it, or if you use only one of the triggers perhaps the secret door opens to a different location; or, perhaps the trigger is just time-delayed

)
- small rocks/pebbles could be scattered in the area, from where small stones or other pieces of dungeon flotsam were crushed in the opening mechanism/hinges/whatever
Some more obvious clues could include:
- PCs hear some recurring noise from the area on the other side of the secret door (dripping, wind moaning, wind chimes blowing, etc.)
- tracks lead right up to it
- someone marked the secret door with graffiti, blood, a
symbol, etc. (but the PCs still need to find the opening mechanism)
- earlier in their explorations, the PCs hear about some distinctive feature near a secret door, and when they eventually find this feature (or at least what they assume is the same feature...), they commence searching for the secret door and its trigger