Sorry for the crappy pic. Im referring to the inner ring of the rotor. It looks like its the only part making contact with the pad.
Car only has 7k miles.
Take the wheel off and inspect the caliper and pad, grease and re-install. Could have a dragging parking brake but this could be nothing to worry about. I tend to see weird rotor wear when pads are not bedded right.
Oh....the OP has messed with the brakes. I don't know about the S550 rear calipers, the S197 rear calipers had to have the pistons wound in to a very specific point to work properly. Some points cast into the calipers and stamped into the pistons had to be lined up correctly.
Maybe something is amiss with the OP's rear brakes.