I am glad I found this site. I was looking for answers to this very issue. We had two pullets killed in this very manner the last two nights. It is disturbing. Our sneaky killer slipped in, ate off the pullet's head and then hid her body in the tall grass. We have so many pullets that we didn't notice at first.It was almost as if it knew to hide what it destroyed.
Not only could it be a weasel, but it could be a river otter. We are going to lock up our pullets every night in the hen house and try to fortify our yard with a more predator-proof fence. We thought it was a good one and the chickens have been fine since we turned them out into it at the end of May. Perhaps the vermin here are "slow" in their seek and destroy process.
My grandpa always used the phrase... shoot, shovel and shut up when he talked about eliminating various predators on the farm.

We live by that code as well.
The problem is that it is so frustrating to lose ANY farm animal, most of us want revenge! I know I do! I really love my chickens and woe to creature who gets in my way!