GS³: Carbonized, leanKernel'd, Philz'd
Put your Kitsune up!
Hard to say. Could be some slight burn in from an app that you may have used in the past that was heavily used (on for long periods) and had some sort of its own boarder. If its burn in, the display is probably still good for a while.
If thats not it, it could be signs of an eminent failure. But it sounds more like burn in to me.