Could this be an idiom meaning 'being excited' or something? If so, is that a common usage among friends?
Thank you a lot!
OK, no one has replied so I will tell you what I think. I'm not sure I understand what Ben meant exactly. But I think it's a reference to a scene, depicted in many books and films, of startled bats flying out of their hiding place and scaring the characters and the reader/watcher. Everyone is "surprised" here: the bats, the characters and the reader/watcher. I don't know which of them Ben considered himself. He must have seen his arse as the cave, but the cave is the only one that's not surprised in the scene.