A house I lived in when I was younger had a secret little passage. My step-brother jumped out from a mirror one day and I nearly soiled myself. Turns out the mirror swung open to a passage under some pipes and then into the electric (or whatever) cupboard in the hallway. You wouldn’t know because of the pipes, but there surely was a way around and through.
Weirdest part was that there was a gigantic bloodstain on the floorboards, near the mirror. I told myself that someone waited behind the mirror and then jumped out and stabbed some unsuspecting soul.
We put false vents in the walls for a pro wrestler to stash his ‘roids in.