This thread is AWESOME
I think most of the time those things can be explained as mice, little bugs too small for us to see them in the shadows, spider webs waving slightly with the breeze but every now and then some cases could be of a more mysterious nature why not.
I had a black female called Mamoru that suddenly would look at a corner up in the ceiling with round huge eyes and then started to hiss, getting all fluffy for no apparent reason. It was really creepy. That was on a different place I used to live. Never found anything that could explain it.
However in here Scully used to "chase a ghost" for a few nights and once I lied on the floor looking at the ceiling and I realized it was a small spider web thread waving, it was too small and faint to notice it unless you really paid attention. I cleaned it XD
Most of the time it has a simple earthly explanation but you have to keep and open mind for the extraordinary. Maybe some of the cases here are supernatural
@Darmom that pic is really interesting particularly because you took it right when your kitty was acting like this
@Marlowe I would love to see that pic as well