While passing through the hallway of a HR last weekend, I walked by a guest who was talking to an associate outside of his room.

Apparently when he returned to his room, it was latched from the inside. The associate asked if there was anyone staying with him and he said "no".

My initial theory was that the front desk issued a room key to another guest, but besides that being unusual, it's also doubtful anyone would stay for more than a few seconds after walking into a room with someone else's stuff strewn about.

Or he might have left the door ajar and an evildoer walked in and latched the door.

Any other possibilities?