19° is not going to kill them, mine are fine through colder temps. Keep locking them up and perhaps even earlier, before it’s dark. If at all possible, order one of those trail cams. You’ll be able to catch any sneaky animal when they come by. However, I do hesitate to say it was an animal because you did not identify significant or even minor injury. Report back to us if your chickens are acting off (inactive, droopy, strange poop, discharge in nose or eyes, etc)