I once had a female coworker who was complaining about how she had walked in on a male coworker using the single-occupancy bathroom (peeing, his back was turned to the door), that him not locking the door was somehow inappropriate of him.

Somebody put a poll up on a white board with the scenario, with question "who behaved inappropriately" with the choices "the person entering the bathroom without knocking" "the person using the bathroom without locking it" "they are both wrong" and "we're all adults here, get the fuck over it."

The tallies were overwhelmingly in the "get the fuck over it" column. But I feel the poll was missing something important: the door had a tendency when locked to stick and leave the person locked inside. We were in a quick-response duty status (as in running to the aircraft), so the person already in should absolutely not have locked it (he was the runner).

You see a closed door to a room (of relative privacy) that might be occupied, you knock. Simple as.