Well, The Naturalist agrees that flying squirrels fit your description. They are not rare, but are rarely seen -- part of the "nocturnal" bit. Like, folks at the nature center usually find (gross-out alert) just the tails, left behind by owls...

As noted by others, a fisher wouldn't be looking for birdseed. Might have tried to eat _you_, instead. They are vicious little beasts, and would be very interested in the resident cats.