This happened a few months ago...I had a handful of different chickens. Two barred rocks, some brown one (I can't remember which kind of chicken she was), a leghorn, and two "Polish" chicks, which were much younger than the others mentioned.
One terrible night, my dog started barking randomly. I thought she just saw some car lights outside, so I ignored her yapping. It wasn't random barking, because she must have noticed something. In the morning, I got up just to hear my brother tell me all my chickens were slaughtered and ripped to shreds. A gruesome sight it must have been, but I refused to look. He told me the details of what happened, but I won't mention them here. He didn't find the leghorn anywhere.
Many years ago I had a chicken who was killed by a stray cat (who we later "dealt with" once we saw said cat), so I could only assume a stray cat or some other wild cat got in the coop, made sport of the poor helpless chickens, and just left.
When the cage was cleaned up, I found the scalp of my poor little Polish chick, which was nothing more than a patch of pale yellow feathers. I threw it away, of course. I wished that I were able to find the animal who killed my chickens so I could put an end to it permanently.
Anyway, I wandered around the yard, then saw something out of the corner of my eye. There was a flower bush in the corner of my yard, and something white was poking out from between some leaves. I went over to see what it was, and to my delight it was my leghorn / white chicken. Her eye seemed to be clawed / scratched, her face a little bloodied, and seemed to be traumatized but she was alive.
After some weeks of care, vet visits, and being taken care of, she's alive and well now. But I'm still amazed how she escaped her coop, which was shut tight, and survive such an attack. But I'm glad she did. I was sickened when I heard what happened to my chickens, and I wouldn't ever be able to look at something that gruesome.
tl;dr
My "flock" was killed and ripped apart overnight by some animal, only one managed to escape miraculously, and she's okay and well.