I had my DS front wheel off this weekend and noticed something I haven't seen before. The rotor had a black "smear" (for lack of a better term) about 1/3 in from the outer edge all the way around the rotor. I took a damp rag to see if it wiped off, and nothing came off. It reminds of something that might be "burnt" on there. Does anyone know what I'm looking at here? Problems it might indicate?