This probably doesn't help at all (sorry ) but I used to complain about seeing "shadow forms" in my peripheral vision. Always in the corner of my eye, always an animal shape, but they were always in black/gray without definite edges. Anyway, I mentioned them to my friend once, and she wondered if I wasn't "psychic" and seeing spirits or some such thing. I've read that hallucinations are more permanent, last longer, that sort of thing. I could be wrong, I'm not an expert by any means.

On a similar subject, I will often hear what sounds like a radio or people talking in another room, when I know for a fact no one is there. Drives me bonkers.