I helped a friend move once and found a secret room in his apartment that the owner hadn't known about. It was hidden behind a funny-looking wall panel, which opened to reveal a depression in the wall that was a couple inches deep. I searched the depression and found a tiny hasp that could be flipped up, which unlocked the back of the depression so it could swing out to reveal the room. Not much in there but really old newspapers, but it was still pretty cool.