hello Elizabeth,
Looks like your phone's back glass has been replaced. It is possible that the glass comes from China and is not an original product - that is, your phone has been modified in some way - using the nomenclature of an authorized service center of Apple. There's nothing inside that would explain the mark. If it is not an imprint of the surface being wiped by the case, then there is only a trace of the suction cup from the production process of the replacement element.