I'm not the OP, but I had the same issue on mine. It also lined up with the edge but I think it was a stuck pixel that ate itself through the outer coating. That's also what Apple said, btw.
Apple replaced the screen, but mainly because mine also failed their IR test.
I now purchased a keyboard / screen protector from Radtech, just in case it really was some mechanical issue. I'm always super careful with my rMBP, but I carry it around pretty much 24/7
Peter