I bought a skeleton key from the hardware store for a house that was about a hundred years old, to test if the locks in my house worked. It did, and for the next couple of months, I'd prank people by locking the door to their room, or the bathroom. Never locking them in, just out, and then when they wandered off frustrated, I'd open it back up.
I've always pranked people, but them not knowing there was a key to their own bedroom is always gonna be one of the better pranks I did.
Dunno if that'll quite fit your goal, but I still have that skeleton key, kept it even when a tooth broke off and it stopped working.