I work in an old converted school building. The township was mostly farms in the early 1900s. By the late 1930s most of the farms were gone and the population was growing. They needed to build a high school. There was a long wall mine that had shut down. The land above the mine was available so it was purchased as a location for the school.
In the late 1960s the township merged into a larger school district and the school was shut down. It was sold. The first buyer was a man who wanted a place to have a nightclub. He opened a disco on the first floor of the building. The disco went out of business fairly quickly and a new owner purchased the building. He opened a different night club in the building. It lasted a bit longer but eventually shut down.
When the second club went out of business, the building was put up for sale. After a very long time the building was sold to a company that refurbished it and remodeled it as offices. Some of the areas of the building were walled off to make it look like there weren't as many empty areas. As new tenants were found, these areas were reopened.
The third floor (where my office is) has always been the last area to be rented. My suite is across from two other suites. Both offices have gone through many tenants in the twenty years we have been here. The tenants have moved out due to a plane crash which killed all employees of the office, bankruptcy (four different tenants), sudden heart attack of the owner (fairly young man with no history of heart issues), death of a key employee, and two tenants moved because they said they never felt comfortable here.
The third floor also has had some unexplained events. One woman told me she was tired of "that girl" smoking in the ladies' room. I had never seen anyone smoking so I asked who she meant. She said the girl was a teenager who always wears a sweater and full, a-line skirt. There are no teens on this floor or anywhere else in the building. In fact, there are only 4 women on this floor. I was repeating this story to another woman on the floor. Her office is directly across from the bathroom. She said she often hears the stall doors slamming. When she goes to investigate, there is no one there. Since her office door is right across from the bathroom, she would see anyone leaving.
There are often the sounds of a large group of people in the hall. You hear lots of people moving around and talking. The odd part is you hear the sounds of footsteps on a hard floor. The hall floors are all carpeted. We have even gone downstairs to see if there are people in the hallway on the second floor. There weren't.
I have no idea if this is related but the events became much more common after the sewer line was installed. When the school was built, the waste water drained down into the mine. It did not connect to the sewer line. About ten years ago the township realized the building was never connected to the sewers. A line was installed which replaced the opening into the mine.
There are lots of clowns about deaths and injuries in the mine. There are none about the school that I have found - but I'm still looking.
I do lean towards a residual in the bathroom. It never varies and there is no interaction that would suggest an intelligance.