I have seen that before, and I did have one hen that was going around, pulling the feathers out of my entire flock of chickens, in that EXACT same spot. I got rid of her, and everybody's feathers grew back. You might think that nobody is pecking her, but if you have the time to just sit quietly and observe, that's usually when you will see the feather pecking bully.......(I'm a detective haha)
One thing a friend taught me, was that, if the entire feather, quill and all is out, the feathers will grow back, but if the quill of the feather is still in, then it won't grow back until the next molt. I think I see new feathers coming back.
Since you mentioned you don't see any parasites, then I'm thinking it was from the mating before you had her, or a hen may have noticed that spot and is pecking it. (Chickens seem to be drawn to bald spots....) What I like to do is put some Blue Cote spray on there, and chickens don't seem to find that bright blue color appealing, so they stop pecking. It really helps.
You could also put a hen saddle on her, to cover and protect that bald spot until the new feathers grow in.