our dog would occasionally go rigid, fix his eyes on seemingly nothing in the room, and then act like he was looking at a person moving around (i.e., moving his head, cocking his ears, etc.). It was really creepy.
I've had that happen with a few dogs too... spooked me. For those who have NEVER had an 'encounter', all these things seem silly - but for those who HAVE had encounters, they are undeniable (although unexplainable).