All of my DLPs had a brownish circle area near the center of the screen when turned off. It is, as videobruce mentioned, caused by light reflecting back through the optics, and is normal. If you're seeing it with the TV turned on though, it's quite possible the screen is damaged. The previous owner may have used an abrasive cleaner on the screen, or liquid could have seeped in between the screen layers. You can call Mitsibishi and order a new screen, but first be sure of your problem.
With the TV off, open one of the side portholes and reach inside with a flashlight, shining it out through the screen to confirm the problem. While in there, check for dust on the lens surfaces as well.
BTW, I once worried over (and tried to clean) lines on my screen for half an hour before I realized they were caused by the Venetian blinds in the room. Light and shadow effects can be that subtle.
"The wise understand by themselves; fools follow the reports of others"-Tibetan Proverb