one of my old BO hens is plucked red and bare on her back. I am sure I don't have a rooster. also some of the old hens are almost naked around their vent. are they picking it out or molting or parasites or ?
She may be getting picked on by the other hens for something that you can't see or detect. As for old hens being bare around their vents, that's normal. A result of laying eggs for a good long while.