Is There a Ghost Behind Me?

I used to work overnight security at a few different sites here in Chicago and one of them was a large corporate office building. One night I swear to God I see someone walking behind me for a second, like it was so real that I almost spun around in fright before I realized it was probably just a trick of the light or something. It was so weirdly subtle that I barely even noticed it, like the kind of thing you don't really notice at first, but when you do, you can't unsee it. Three nights, and I noped out of there, settling for sucky unemployment before I could get another position.