Have you got a smart cover? I get lighter lines down the screen where the folds sit on mine so it looks like there are dark patches down the middle where the two center panels of the cover sit... give your screen a good clean and they should go away for a while.
...actually, yeah. And the two lines just about match up with where the left and right folds sit.
It's not an incredibly big deal since I can only see them underneath an extremely bright light while the screen is off, but it did make me curious.