I've noticed something that I consider to be strange about my 27" iMac - on the top right corner of the display, there is a greyish blob, as if the colours in that area were stained. What's weird about it though is that it does not always appear. Right now it is completely invisible, but it seems on some days it is very obvious, on others, completely invisible. Does anyone have any idea as to what may be causing this? I have a suspicion that it's either heat or perhaps humidity related. Sorry if this has been said before by other users. Any ideas?