Is it an aluminum iMac? You could lift the glass off and try see if it is the LCD or not.
By the way the glass is held on with magnets. I just grabbed a $0.99 1.5 to 2 inch suction cup from Ace hardware (the kind you use to hang up stuff off a mirror or windows). Layed the iMac on its back. Suctioned to a corner and gentlylifted (the glass is quite thin).
my first thought was if its not under warranty any more and you get the glass of ...if you are lucky and its not a preasure mark then it might be , but only if you are lucky ,some washing up liquid that found a way between screen and glass
Ok this is very strange ..
I got up this morning to find no marks what so ever any ware on the screen. All gone. Disappeared .
They did not look like some thing that would disappear ... Any ideas ??