Strange mark below screen

[Dexter17]

Nearly two weeks ago I bought a new xz2 compact from a tmobile store, and have been using it since. Looked after it, never inflicted any sort of pressure of stress to the screen, and I just noticed today there's mark in (or beneath) the glass screen. This is only in the middle of the screen, and judging from the unsymetrical and odd look of it I guess it isn't intentional either (or anything beneath the screen). I have only really noticed it today whilst sitting under a bright lamp, but now I know it's there it's driving me crazy. You cannot feel any mark or scracth on the screen, and when put in different lighting conditions the look of it changes. 

In this image the other perpendicular line isn't a mark but just my tablet reflection

[Lord_viridis]

Have you spoken to T-Mobile?

That's the first thing I'd personally do based on its age