I have a 27" iMac that is about 4 years old. Lately I've noticed a discoloration in a small area at the bottom of the screen just left of center. There's a brown "stain" if you will. Almost unnoticeable unless you look for it. Plus - and this might be my imagination - but the screen seems to feel slightly warmer in that area. This discoloration may have been there for a while before I noticed it, not sure. Any ideas what may be causing this and is it cause for alarm? The machine seems to work fine otherwise. Thanks.