Back in 1996 I was working in a mortgage bank. It had cubicles with "hallways" along the walls and between the cubicles.

I was taking my 15 minute break at my desk with my head down on the desk resting when I felt someone tap me on the arm as if to get my attention. I opened my eyes and there was no one there. I looked down the "hallway" between the cubicles and the long wall of windows. Nobody was there. There was no way anybody could've gotten from my cubicle to where I couldn't see them in the few seconds between when I felt the tapping and when I stood up. I mean- there was nobody there!