The Weasley twins concluded that he probably had a broomstick. Not being able to see unanchored inanimate objects like that was one of the Map's limitations, after all. Although there wasn't any sensible reason for a teacher to have a broomstick just lying around in their office, there was no other way to explain his apparent failure to experience injury from falling five stories. For that matter Professor Quirrell seemed like the type to be terrified of broomstick riding, but it wasn't like he was immune to gravity or something ...