I vote for flying squirrels. At least one species of flying squirrel is very small, and can get in surprisingly small holes. We had mystery animals in the attic of a house surrounded by trees years ago. I went around and around with binoculars looking for where something could get in. the soffit vents and gable vents were screened, but I finally saw a hole in the corner of every gable vent about the size of a quarter. I put heavier screen over them that they couldn't chew through, but we were still hearing them.
I went up on the roof and found that a section of ridge vent had come loose and had a gap of just about a half inch they could get under. Screwed it down tight and we never heard any more varmints in the attic.
