Welcome. I would look her skin over closely and especially around the vent for any signs of lice or mites, and their white egg clusters or darks spots of dirt. Does she roost or sleep on the floor? If she is sleeping on the floor, she may be irritating her breast bone. Broody hens may start pulling their own feathers on the breast and abdomen, but that doesn't look like the amount that a broody has. I have a couple of hens who have little bald areas on the chest who are not broody, so it may be nothing at all.
She sleeps on the roost so I don't think that's it. She hasn't been broody... it almost looks like an old scar. I should give some background. We took her (and some friends of hers) in after they were abandoned in a shed in the woods this winter. We really don't know her history. I didn't notice any mits earlier but I will grab her again tomorrow to be sure. I think it's been there since we have had her. I have noticed the look of the lack of feathers since we have had her (I just thought it was How her feathers laid)