Reynard
aka Ian Eller
Yeah. I also like the device of having a trap or secret door early in the dungeon left open/fired, maybe with a body lying in it, as a clue for the PCs to be on the lookout for more. The Lost City is a classic module which does this.
Edit: I think a fun variation on this is to do it in a large (maybe mega-)dungeon several sessions in. Having a door suddenly there, ajar, that they've walked past a bunch of times without knowing it is great bait and a great reminder that there are secrets they haven't found.
Clues to secret doors come in different forms; leaving them open is a clue and it also makes the dungeon seem alive, so to speak. I think it may be smart to think about a secret door as the clues that reveal it. Each secret door should probably also have a workaround.
Another good one is having the bodies of guards or whatever the PCs have already killed disappear, with the only clue being a drag mark ending at a wall.