MacBook Air 11" (mid 2011) - yellow spots appearing on lower left part of screen

Hi, on my screen appear yellow spots that look like finger prints but the effect seems to be originating from inside the screen, below the surface. I went to the Apple Genius bar and they said this was reported for iMac and iPhone (with different causes). But when searching about this online for the MacBook Air I couldn't find anything.

I'm a non smoker and nobody smokes around this computer. I live in Paris, have the window in the other room (a bit) open, but there's no Smog or weird atmospheric condition that would seem to be the cause.

The funny thing is that sometimes the spots are evident - usually after longer work on the computer - and sometimes all seems OK. I had a meeting at the Genius Bar and they would have replaced the screen -- however when I presented the Mac the spots were not showing. Today they showed up again at the same location - bottom left.

Here's a picture: http://bit.ly/MDEq16

Am I alone with this? Anybody else have experienced this?

-martin