I think it could be her butt. As someone mentioned, there are many people with really defined back dimples etc that could be mistaken in photo for a navel, and plenty of people have flat bums like that. I've certainly seen plenty of people with backs that are that defined, and who look like that bending over (assless, weight ass to hip to back ratio and definition) all without photoshop.
However, these companies just love excessively photoshopping already sufficiently skinny and cellulite free people until there isn't a blemish to be seen and their calves are the size of my arm. It IS possible they flipped her ass and tried to turn in navel into a back dimple etc.
Also, the top line of the underwear seems a little unusual, and it could be that the picture that they used with the underwear down was of the model bending over at the waist. They may have then removed her original upper body from the shot and meshed it with one of her standing upright.