In '99 I was working as part of a project team on a hospital campus. Our building was an old historical building with a new building added on to it. There were five above-ground floors and a central staircase between the old and new parts. At each landing or foyer area of the stairs you could go either left into the new building or right into the old building.
The old part had been psychiatric hospital wards, with a not too pleasant history. Our project office was on floor B, almost directly off the stairs foyer. It was essentially a walled off section at the rear end of what had been a ward. Beyond the partition wall the ward was still as it had been in the past, a huge open space with vintage iron beds lined up down each side. The space was used for nurse training now and had also been used as a movie set for a film set in the 70's.
Our toilets were the original ward toilets. You walked through a door in the side of our office, across a short hallway to the toilet door. To your left the ward opened out. It wasn't dark during the day and you could see most of the room from the hallway.
On this particular day there were three of us in the office. I left to go to the toilet. The toilets and sinks were all unbreakable metal and the mirror above the sinks was also a sheet of polished metal. There were 5 or 6 stalls, three toilets along one side. The sinks were on your right as you walked into the room. I went into the first stall on the right closest to the sinks. While I'm in there I hear the main toilet door open and someone walk into the room. There had been no-one in the ward area so I figure it's just one of my co-workers from the office. They walked past my stall and into the end one on my side. I hear the stall door close.
I leave my stall and go to wash my hands. Then it hits me - I turn back and sure enough there are no closed stall doors. I think my head's playing tricks so walk back to the stalls to check - absolutely no-one there. Washed my hands as fast as possible, while looking over my shoulder the whole time, then high-trailed it back to the office.
I kind of laughingly tell my co-worker what has just happened and she goes pale and says "You've heard it too?" She had had exactly the same experience. She also said that when she and my boss had been working back of an evening they had both had experiences when they could hear people moving around in the ward on the other side of the wall and talking. On different occasions they had called security up to check on it but there was never anyone there.
I made a point of never working late and only using the toilets in the new part of the building after that.
TL;DR : Worked in an old hospital building with a haunted toilet.