One of my sites was a closed down hospital, which some movies were filmed in.
One of the elevators would spontaneously stop or be called to a floor. You could be riding to another floor, or you could be walking by, and the elevator would open up. Then there would be this blast of cold air that would exist for a few seconds before it warmed right back up to what was normal. To make it even weirder was that it could be in the summer, and the HVAC would be turned off when it would do that.
At the site I worked with Bass, SWAT, and Grandma, we had three ghosts in one of the buildings. Supposedly, there had been body dumps left at the building, and one time a mental patient got onto one of the top floor balconies before taking a header off of it.
The three ghosts I termed "The Secretary", "The Giggler", and "Footsteps". The Secretary, sounded like somebody the next room over taking a phone call.
I spent over an hour and a half on the floor looking to see if anybody was about one night. After speaking with some other night employees, they said they heard it, and thought it was another co-worker and forgot about it.
The Giggler sounded like a little girl playing. She only showed up when a) you were the only person on the floor, and b) if everything was working OK. One time when I didn't hear her on a floor, and I knew I was the only one there, I found a stairwell door propped open. Another time, another employee working late didn't hear her and found a refrigerator full of medical samples burned out. After fixing both this, she came back.
Footsteps was the only one to truly freak me out.The floors in this building were laid out in an H pattern, two hallways with the elevator hallway running between them. This floor also had hardwood floors. One night, I patrolling and was in the elevator hallway. I heard footsteps of leather soled shoes on hard wood, so expensive shoes. The footsteps went down the hallway right in front of me. There was nobody accompanying the footsteps. I never checked that floor again. I just made sure the doors to it were locked and moved the hell on.
May your late night custies not be as spooky as some of mine!