I bet the key to the other door opens this one too.
For fire safety there’s a key behind some glass that opens a back door, and that passes through a storage room that also contains spare keys. You can see them through the keyhole! But, should you break the glass? That seems wasteful.
You keep looking around. At the top of some stairs you find another locked door that leads to the upper story of the house, where the owner lives. When you look in the keyhole, you notice it’s dark. It appears that someone has left a key in this door, on the other side.
You also notice that the faceplate over the keyhole only has one screw in it. Has someone taken it apart before you? You don’t have a lot of possessions to work with, but you poke around the laundry room and find a dull knife. It’s just enough to turn the remaining screw.
It’s also enough to turn the entire locking assembly once you get the faceplate off. You do so, and hear a loud “clunk” as the lock disengages. You open the door…
… and discover that the owner has left a complete set of keys to the entire house, including all the rooms, hanging in the lock on the other side of the door.
You unlock your own room, fetch your key, then place the keyring back where you found it, and screw the faceplate back in.