I have a theory. Just yesterday I put out mouse poison because I had seen a few of the varmints in the coop. I'm thinking possibly she died of natural causes and the mice gnawed on her head??? I mean that could make sense considering there was no way a coon could get in, I checked for entry points and found none ( my run and coop are surrounded by 6 foot chicken wire and have a 1x10 buried under the base of the fence) and no blood or other signs of a struggle. Just a theory
If she was sick the rest of your flock will team up to more or less put her out of her misery. I say that because the comb is one of there go to places if it is an attack from other chickens. Not so sure about the beak part though. That's a mystery to me