No predators. Locked coop. Prior to dying my son said she was just failing and he didn't know why. He has over 30 chickens so he didn't notice it until just before she died. Yes, I thought that looked like a maggot too but I don't know how since it's the middle of winter here yet and maggots come from flies, right? It's possible she was dead in the coop for up to 24 hours since he just goes in there once a day do to chores and check on them.
There have been no outside influences (new chckens) in the coop since August and he hasn't been in any other coops to pick up anything.
I'll have him check over the other chickens closer tomorrow.