This happened in 1978 in lower Manhattan. I used to work at a gourmet deli on West Broadway. I worked nights and we closed at around 1 am. For an extra $20 I had the job of buffing the wooden floors with a buffer machine every Friday night after we closed.
Above the deli was a sort of a club/restaurant where my boss tended bar and his wife hosted. I think it was not legal and it had the feel of an after hours club to it. Once we closed downstairs, everybody would go to the back and go up stairs to the club.
I was downstairs this one Friday night, totally by myself doing the floors. After about 45 minutes I finished and went to unplug the machine. To get to the plug I had to pass the door to the basement. There was no real door that closed, that had been removed, and all the lights were on down there in the basement. As I passed the opening I glanced down the steps and saw someone standing there at the bottom of the steps looking like he was just about to climb up the stairs. I passed by after seeing this maybe one foot and unplugged the machine. I immediately turned and went downstairs. I was thinking that maybe someone from upstairs had gone down the basement for supplies for the club and I hadn't heard them because of the noise the machine made. So I went downstairs to find them and tell them I was closing up. I went into the room to the right of the stairs, where all the soda and Pepsi and 7up were kept. No, not there. I walked down the hall and opened the walk-in refrigerators. No one in them either. I walked further down the hall looking into this corner and that corner thinking where did this guy go? It wasn't until I got to the end of the hall and I was facing a brick wall in front of me that my thought was "this is a ghost." All the hairs on my neck and head felt electric and I turned and ran out of the basement. I grabbed the machine and slammed it into the closet and turned out the lights and went up the stairs two at a time to the club.
I was sitting at the bar and my boss, who was the bartender, comes up to me and says, "Hey, you're all white. What's the matter?" My boss was a guy who I consider to have no sense of humour. I said to him that he wouldn't believe me if I told him. But he said, "Try me."
I told him the story and he asked me to describe what the guy looked like. He told me then, that when he was building the bar, late one night, all by himself, he looked up and saw a guy at the end of the room watching him. My boss did a double take and looked down to grab his hammer for a weapon, a second of an action. When he looked back, nobody was there.
One other side note. Before they installed an alarm system, they hired a friend to sleep in the kitchen upstairs at the club. One night, while he was sleeping, something grabbed his foot and shook it hard. The friend grabbed his clothing and went out the fire escape and wouldn't do that job anymore.