I'm sorry for your loss. This exact same thing happened to one of my 5 mo pullets last September. I found her exactly the same way - decapitated, no blood or feathers scattered about, otherwise intact. It was 11am on a bright, sunny late summer morning. I'm quite certain I heard my girl when she was attacked; I had heard a loud squawk coming from the back corner of our property. It backs up to the woods and we also have some tall chestnut trees back there. The girls used to like to scrounge back there. Anyway, I didn't think too much about the squawk I heard, just thought the girls were squabbling again. I was on my way to take one of my dogs to a vet appointment so I didn't have a chance to do a quick roundup and inventory. When I returned home I began the roundup and realized my Buff Brahma was missing. I found her a few minutes later, in the back corner next to the fence. No feathers or anything, just her headless body, as if her hear just popped off. I took her away to bury her. Later that evening I saw a large bird swoop down and buzz over right where I found my BB; I couldn't tell if it was a hawk or an owl. Seeing how my girl was presumably nabbed in broad daylight, I'm leaning towards a hawk but I know that there is also an owl that lives in the woods so I can't be certain. My mom asked her friend, who is a poultry keeper at the zoo, his thoughts and he felt it was probably a hawk. Apparently it is not uncommon to see this and chickens can be found in this condition if the raptor goes for the head/neck. The weight of the chicken's body plus gravity, coupled with the raptor pulling in the opposite direction, well...you get the picture. My apologies for the gruesomeness.
I wasn't surprised this happened to my BB. She was rather shy and would be so engrossed in scratching that she wouldn't notice her sisters had moved on and she would be all alone until she rejoined the flock. However, I was still rather upset, as this was my first loss, and my BB was becoming more sociable with me, and she had started to lay. She also had really cool feathered feet and it looked like she was wearing Uggs. But this is all part of the circle of life so things like this will occur and when they do, we can only hope their demise is swift and painless.