A fox won't stop at a bird or two when its got kits to feed.The have a small stomache and can't eat but a small amount at a time and carry them all back to the den or bury to eat later .They're numerous posts here where they killed the whole flock when the birds were small and the den wasn't...
I recall her posting she found a fox den nearby right after the first attack then it happened again. She also caught a fox with a hen in its mouth but yelled and it ran off (dropping the hen) .Foxes are cunning and smart .If a fox messes with a slide bolt lock long enough it can open one. So can...
I recently spotted a pile of feathers on the lot beside me and mistook them for a bird at first. My guess is they belonged to one of my neighbors chickens since he's the only one close by with white chickens. So it IS possible for a predator to carry someone else's chickens to your house or nearby