That is a wild story and I am glad you found out what did that right away, it would drive me nuts the mystery of it!
I have witnessed a black rat snake climb straight up a vertical brick wall in a corner, but still. They are insane climbers!
I'm *amazed* I haven't seen a snake yet, but our hen house is exposed and not near a treeline or anything remotely safe at all. Honestly I think this is why I just don't see predators at all. They gotta get REAL exposed to come at it and there is plenty else to eat around here. Lotsa eastern cottontail bunnies, for one lol