I've seen it before on both my old S1 and the S3. I've only ever seen it when using the phone in the dark, when you get a black screen pop up for a couple of seconds (start or end of a video, or going into an app). It's definitely there, you can only see it in the right light.

I've also seen reports of the same 'problem' and read that it's perfectly fine, it's just the way AMOLED works.