Definately not mice. Mice generally don't make noises, rarely even a squeak. And, believe it or not, they suck and jumping and are very slow cautious climbers. If a mouse got in through a home's basement or first floor it may never make it up another floor. Bats hibernate at this time of year. It could have been squirrels or a bird. Did it sound like a bird?
Don't freak out yet, you usually don't have an infestation the very first time an animal gets into your house. It was probably accidental. You do want to get them out quickly though and block their way in because you don't want a squirrel chewing a wire and starting a fire. First thing you do is look for places an animal could have gotten in near the roof. Vents are usual culprits. If you block the entrance, they won't get back in. You can set a trap to be sure nothing is left, but it probably will remain empty.