I seem to get this a lot... :-(
Thrice in a hotel in Southampton booked by work.
1. An empty room save for a very expensive cabin / laptop bag left on the bed. Owner had just booked the room and left it there. I insisted the hotel call the police to remove it. They reluctantly did.
2. Arrived quite late and walked in on a single woman with kids who was understandably very upset.
3. Walked in on a youngish couple. Got very red faced.
I refuse to stay there now.
In Belfast once, with a bloke just out of the shower.
I now knock on the door before entering and always use the security locks.
Mind you I wonder what would have happened if someone had walked in on me when I was in the States taking an angle grinder to a laptop's HD to destroy it on the instructions of my employer (posted elsewhere on this site).