Very sorry for your losses. Any of the tree climbing predators could do it, bobcat and raccoon come to mind first. Owls also hunt at night, and will grab sleeping chickens from trees. You mention you have never seen raccoons, but most of the time you won't see them. I have a couple of motion sensor trail cameras set up around my coop, and I rarely see animals, but the camera catches raccoons, oppossums, bobcat, coyotes, deer and bear. I would set one up so you can find out what is coming and getting your chickens, and make sure they spend the night inside. Bad things happen at night.