How would an animal get on your roof?

I was sitting on my roof last night for an hour or so at like 3 am and i planned on watching the stars for a litte while longer maybe even watch the sun come up because im 18 and have nothing better. when all of a sudden on the roof above me i hear what i think is maybe an animal. All of my cats are inside and my window is shut so it could be them. So i hurried up and opened my window and crawled back inside. the problem is though that there are no over hanging branches next to my house and the nearest tree is like 30 feet away. Also there are no vines connected o my house as ell. and my house has inyl siding on it so how would it be possible for an animal to get on my roof.