Wall Lock

It was a normal looking lock, set in the middle of a brick wall.

The two of them found it while exploring the supposedly haunted abandoned school, and they both agreed that there had to be something very important behind it.

So, they left, and came back three nights later with their friend who was handy with a lockpick.

Took him five minutes to pick the lock, and the hidden door inside the wall swung open.

Revealing a bunch of water pipes.

Dejected, the three of them went home, none of them noticing the other door hidden behind said pipes.

(Many thanks to Sandy for assistance with this one!)