Maybe it's just the elves

Several times a week, I'll see some movement out of the corner of my eye. Usually it seems like something smallish and gray is moving about there. I turn, thinking it's a cat or squirrel or some other small animal, and nothing's there. I'm wondering if I should be worried about it, or if this type of mild hallucination (?) is common. I'm in my thirties, in good health as far as I know. I do wear glasses but am seeing these things outside of the range of the lens. It's always very indistinct--usually if I'm at home when this happens I assume it's one of the cats walking into the room and turn to look and...nothing. It doesn't bother me all that much, but at the same time it's kind of weird. It usually happens when I'm really tired, so I'm wondering if it's a hypnagogic-type thing--like maybe I'm going into a microsleep while sitting at the kitchen table or whatnot.

Googling hasn't seemed to yield much info, and I was just curious if this happens to other people a lot, or if it might be a sign of something I should be concerned about. I'm not sure when it started, but I've been pretty conscious of it for the last several months.

Thanks!