Grey smudge visible on screen when its black

[ascender]

With all the screen related issues, I've lost track if this is a known one or not, but can anyone remember? When working with dark colours, I can see a grey mark on my screen, a few inches wide by two high. It looks like a smudge mark, but is behind the glass. There seems to be a small one at the opposite corner as well.

Does this mean another trip back to Apple or is there an easy fix?

Thanks.