Mineral deposits on the bottom of the oven cavity
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.