Hi and welcome to the forum.
Do you mean on the inside of the back cover, maybe you could provide a picture?
The original transparent back cover can be replaced with an opaque one which has a slightly raised rubbery surround which I think is there to push against the battery.
Do you mean the little sqaure to the bottom right, not visible on the opaque cover. It appears to be the NFC antenna so it may be an issue if use NFC, but it is only a booster antenna.
If you have problem you could probably replicate with metalic paint or gluing small bits of aluminium foil etc.