It really could be anything. A good bonfire party is common this time of year. Chickens alert to all kinds of things out of the ordinary, could have alerted to an intruder, the drums, an animal, something knocking around in the wind. Definitely better safe than sorry, and adding a lock to your coop and run is a cheap and easy measure to take. It's a bit of leap to go from a noise in the dark and a fire to animal theft and sacrifice, though anything is possible. I hope your chickens are safe for another October. My husband asks that I keep my black cat indoors this time of year as well for similar reasons. I don't know how realistic someone stealing and messing with my cat is, but again, keeping him inside for a few weeks is an easy measure to take, plus I can't deny that the coyotes have been out in full force lately, again, better safe than sorry.