This is a like a story within a story.
I am an architect by profession. Soon after graduating, I landed a job in a small firm. How small? Think ten people altogether. We worked in a building that was reputed to be haunted. To make matters more interesting, the office was on what was supposed to be the thirteenth floor. (The building administration just numbered it fourteen so as not to scare people.)
The boss was middle-aged and already had a steady stream of clients. I got the "bread and butter" clients and that meant I had to do a lot of overtime work. I usually stayed maybe four or five hours after everybody else had finished for the day.
The boss told us to save on electricity whenever we could and that meant, unless it was really hot, no air conditioning for overtime work. My work station was arranged so that to my left, about ten feet away, was a window type AC unit, and to my immediate right was the kitchen. (That means I was comfortable during the day, and it made it easy for me to snack while working.) The kitchen wasn't exactly a room - it was more of a big alcove. The boss installed two heavy sliding doors to close it up and make it seem like a separate room. He bolted one panel so it would remain fixed while the other one was left free to slide open and closed. The rollers and rails were on the top frame, so with a little bit of effort you could actually push laterally on the non-bolted panel and cause it to sway and slam back on the fixed panel.
One particular December evening (I think it was about seven or so) I was finishing some files for a big meeting the next day, The AC was off, and all was quiet. I felt hungry and was about to go to the kitchen when I saw the sliding panel swing inward into the kitchen and slam back onto the fixed panel. It made a loud noise and the whole thing was a bit disconcerting.
My first thought was that I had forgotten to turn off the AC, so I went over to the window and checked the unit. It was off. I looked around under the desks, thinking maybe someone had left a fan on. There were two fans and they were both switched off. I remember feeling more annoyed than anything else, and the thought of ghosts or anything supernatural didn't occur to me until the next day when I told the boss's wife about it.
Let's call her Annie. She was about my age, so we got along well. Before I came to work there she was the one usually doing the overtime work. Her husband was usually out meeting with clients, and he would come pick her up on the way home. She did the administrative and some clerical work so she would stay after hours maybe two or three times a week. When Annie heard my story, she was surprised I wasn't scared and began to relate what happened to her.
Annie was alone in the office, doing the salary computation on her computer. She paused for a while, wrote some checks; as was her habit, she would turn off the monitor while she did other things. After she was through with the checks, she then turned back to her computer. She was shocked to find a reflection of a man on her monitor. He was walking behind her from left to right, and from the way it looked, he was maybe two feet behind her chair. What added to her fright was that the man paused near her right shoulder before moving on until he disappeared from the screen.
She screamed (naturally) and ran out to the outer office. She picked up the phone and called the guard at the lobby for help. During the few minutes it took for the guard to arrive, she remained near the door and just looked at her feet. She was too afraid to look anywhere else. The guard came up eventually and she told him what happened. He was sympathetic as he helped her switch the lights off and lock up the office. The guard had been there the longest and has probably seen and heard things too. Anyway, she stayed in the building lobby until her husband came to pick her up.
Needless to say, she resolved never to stay after hours by herself again.