VIMH's comment on an unmarked restroom that is largely unknown reminds me of a story from my college days.
Since my final project involved computational geometry, I was granted a key to the rooms in our department which had the computers with fancy graphical displays. (The term "fancy graphical display" is a relative one, mind you. By today's standards they would be pretty lame.) Use of the computers in these rooms was normally reserved for faculty and graduate students. During my wanderings through the department building, I discovered that there was a small storage room in an unused corner of the basement that contained not only the boxes piled high, like you might expect, but also one graphics display terminal.
I was pleased at my discovery and even more pleased to discover over time that nobody ever came to visit. I had stumbled across the forgotten computer room.
After a few weeks, I moved in a small tape cassette recorder (that being the fanciest audio technology I could afford at the time) so I could listen to music while I worked. Rachmaninoff's Third Piano Concerto became the mental soundtrack to my final project. Initially, I stowed the tape recorder in the corner when I left the room, but I gradually became lazy and just left it on the table next to the computer.
This is normally the part of the story where our hero's casual mistake leads to his downfall: A custodian discovers the tape recorder, reports it to the administrator, and our hero is kicked out of the department for misuse of school facilities.
But that's not what happened. As far as I remember, there was only one time another person paid a visit to the overlooked computer room while I was working in there. He jiggled the door handle, found it locked, and waved apologetically. (He was probably not even somebody authorized to be in the room, because if he were, he would have had a key.)