Tap, spring and even filtered water has dissolved minerals in it that will remain in your oven after the water evaporates. These minerals may leave a ring that is likely deposits of Calcium, Phosphates, Nitrates, Sodium, Potassium and Chloride. A reddish hint in this area is typically rust in the water supply.
Most of the time this ring can be removed by wetting the surface and using a non-scratch sponge. If the problem persists; a 50/50 mix of vinegar and water will remove it.
IMPORTANT: Make sure you completely wash the surface with pure or distilled water after you clean the vinegar/water mixture up to prevent damage to the oven coating.