I have to sit with my back to my office door due to the (unfortunately permanent) arrangement of furniture in here and it drives me batty because people go by and say hello to me and I have no idea who it is because by the time I turn around, they're gone.
But I was eating lunch at my desk and had my mouth full and didn't want to turn around when I heard someone say, "This is a really interesting room." Then the mystery voice said, "This is my daughter," and I nearly choked on my food because my mother died 8 years ago!! !
Turns out it was the new receptionist showing her daughter around. I don't know why she thought I could see her without turning around.