Sounds like a hawk to me. They can be nowhere in sight and bam, in an instance there it is. I let my girls out to range and went inside for a few minutes, I never leave them out unattended (well, not at all after the incident) and I came back to a hawk who drug her out from a bush. The slightest movement of me peaking my head around the corner flew him off my girl. She didn't have a mark on her, obviously the hawk broke her neck and that's what killed her. Hawks get spooked easily.
Soooooo sorry to hear of your loss. I'm in predatorville here, coyotes got my daughters rabbit a few months ago.
A DOG! Dogs ravaged my chicken house a few years ago when I failed to lock the door and killed lots of my chickens and all my guineas...didn't eat one. Just tossed, shook them and broke their necks, one after the other.
It will be probably be back for more "fun and games" and may bring some "friends". Lucky only one hen was killed this time. Sorry for your loss.