HoppyJeep

On my 2012 JKU I'm developing a 5 inch dark line on the grayish inside. It runs from right to left and is slightly indented. It is about 4 inches back from the very front of the top. Looks like a wear mark. You can faintly see a line developing on the exterior of the top in the same area. Nothing touches this area when the top is up or down. It's only been on for 2 summers. Hard top in Winter. Any ideas what this is or whats causing it?

Hoppy's Jeep