Maybe they used rubbing alcohol to clean it - or, you could try to clean it with rubbing alcohol. Apple says that the iPad and iPhone screens should not be cleaned with rubbing alcohol or any other chemicals because it can take the anti-fingerprint coating off - but I have no idea if the MBP retina screens also have this coating. Anyways, rubbing alcohol usually works removing things like this, but like another commenter said, it could be something from the factory that's still covered under warranty. If you want to take a chance, try rubbing alcohol. And then contact Apple if this doesn't help.