I was watching an episode of Stargate: Atlantis recently that had an interesting idea (IMO) for a secret door.
There is a hallway with a solid wall at the end.
As you walk to the solid wall at the end of the hallway, there are 3 lamps - set about 5 feet apart (one after the other - along the way to the solid wall). If you touch each lamp 1 after the other, as you are walking, (if you stop it doesn't work) and THEN walk "into" the wall, the wall is not solid (even though it still looks solid) and you walk into the next room.
@Charlaquin ,
@GMforPowergamers ,
@iserith - or anyone who wants to really.
If you were DMing, how would finding this "secret door," likely, work in your session? How would it look?