Marlene, same thing has happened with me before. I'd be working and through my peripheral vision, on several occassions, saw what appeared to be a rat run by, I'd quickly turn my head to see it and never did. Or I'd think I'd see the shadow of someone approaching on my right side and turn and no one would be there. OTOH, often someone would approach me, I'd be unaware until I jumped half way across the room because they startled me.