Yep, if there was critters I'm sure I would have noticed by now and there would probably be other misc stuff in there like feces, branches etc. I did have mice in my attic last winter though but I took care of them, and I had put peanuts in my basement just to monitor if they found a way to get there, and the peanuts are still there.
Though, this house is like 40 years old, I'm suspecting that's not just my cat.
Mine would get like that with 3 big dogs if they were not cleaned once in a while. Returns on the floor so it acts like a vacuum.
Funny but true story... one day I was making some duck decoys with EPS foam beads and dropped some on the floor. Those little devils zipped 10 foot across the floor and right into the return vents from the suction.