It was a locked family bathroom. Probably every time they came by to clean it was “locked” so they thought someone else was in there. Since no other customers were going in or complaining about the bathroom’s tidiness, no one thought anything of it.
Probably change their processes, so that those bathrooms are checked every night now.
That is what I was thinking, the “family bathroom” is not the one with the stalls. I can see how that one would be overlooked if the door was always locked.