The sensor is on the inner pad you can't see with the wheel on; they Make a very cool felt gauge though for checking and with open enough spokes you can get around the rotor for a measurement.

With a mirror and sometimes with a phone you can get a decent look at the inner pads.

Front left right rear are the sensor positions

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)