How about a hb100 radar sensor, small enough to go in a mirror.
Heres an instructable done by someone who's sussed the protocol used by bosch ultrasound parking sensors, not much use at the range you want, but interesting.
https://www.instructables.com/id/Hacking-Automotive-Ultrasonic-Sensors/
Heres an instructable done by someone who's sussed the protocol used by bosch ultrasound parking sensors, not much use at the range you want, but interesting.
https://www.instructables.com/id/Hacking-Automotive-Ultrasonic-Sensors/