Mice could be a culprit, but they usually don't go up 15 or 20 feet in the air to get in your house. You could also listen for the tell-tale "thump" of the squirrel smacking the side of your house or roof when they land.
Bats do too have pelleted poop. I've got them in plain view in my shed and the turds look like large mouse pellets.
You've got flying squirrels, I would bet money on it. They're cute little buggars. I'd leave them in there, just b/c that's the way I am. I have a shrew that lives in my...
Yep-- especially if you are in a wooded/partially wooded area. They make squeaky noises, and have little tiny poopies. If you watch the hole at dusk, you might get to see whatever it is. Do you hear things scratching around on the ceiling? Bats don't run around, you'd just hear squeaks.