When I was at high school based in a very old mansion house we measured the building to make a scale model but one wing the room dimensions didn't match the footprint, judging by window positions we concluded there was another room off of the deputy headmasters office behind built in cupboards, to cut a story short the odd job man cut a hole in the wall to find a room of some 10-15m² and the light was on. One of the teachers had been there for decades and said it had not changed in his time. There was also a window covered with ivy but the glass and the light were so black it wouldn't have been visible for years
