I get that, too, and it's freaky, because they move yet they don't move. The other night I was outside smoking cigarettes like usual, and I glanced over and the door mat was moving, like a few inches at a time in one direction, yet it never actually made any 'progress' as far as getting to a new location. But I was seeing it move. And it was the only thing moving. So it's freaky on two accounts because I swear I see it moving, yet logically I can conclude that it's not moving, and because my brain will have randomly picked one thing in the environment to be moving and nothing else.

This happened to me even when I was a little kid, though. Like the smoke detector on the hallway ceiling, would be moving towards my room for hours, completely freaking me out, and yet never actually getting to my bedroom.