VISION ultrasonic sensors use a small 44kHz transmitter and receiver to create an ultrasonic field in the vehicle. Even the smallest change in air pressure will trigger the sensor. But unlike RF field sensors, ultrasonic waves do not exit the vehicle and therefore will not false trigger from someone walking past your car while parked.
     The 318-04 also acts like an audio sensor in that it will detect glass breakage; but unlike audio sensors, the ultrasonic detector will not false trigger due to high pitched sounds like squeeky bicycle brakes. The 318-04 also replaces "current" sensors which watch the dome light to see if a door opens. If a door is opened even one inch, the 318-04 will trigger due to the change in air pressure.

Ultrasonic Sensor