Just thought I'd close this thread.
The front brakes have one sensor on the right, inside pad. The inside pads wear more than the outside so a visual of the outside pads is not adequate.
Mercedes knows their cars, the pad with the sensor was worn the most. I would guess I had about 1 or 2,000 miles before metal to metal when the warning first appeared.
Dan

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2012 E350 Bluetech