Similar thing happened to me a few years ago. An acquaintance of mine lived in two different studios that shared a bathroom and a hallway. So one morning I wake up and I hear said acquaintance in the bathroom. I wait for a bit and go out to piss behind a tree in the corner of the lawn at my apartment complex, and on my way back to my room I notice some guy I don't recognize outside on his phone, but don't think much of it cuz this shithole former motel I have lived in for almost 10 years at this point, has a pretty high turnover of tenants. So I don't think much of it and go back into the hallway.

This is when my acquaintance emerges from his room since he heard the door open. Now I am not immediately alarmed because sometimes one of us will turn on the shower and then forget something and go back to our room to get a towel or underwear or something.

Acquaintance looks at me, then at the closed bathroom door and his eyes go real wide and he puts his finger to his lips.

Turns out some tweaker that knew one of the people that used to know a former occupant of one of the rooms had come by, found the front door unlocked, and just casually gone in and used the bathroom. It was her friend that I had spotted in the parking lot and had dismissed.