There are 2 types of technologies built into parking sensors. One is ultrasonic which, as the name implies, uses sound to detect objects in the path of a moving vehicle. THese require a clear view and thus it means drilling a hole in the bumper to fit them. They detect only most objects and may not detect flat or slim items like narrow sign posts or cables.
The other is electro-magnetic which , also as the name implies, emits an electro-magnetic impulse which detects objects. It relies on the vehicle moving slowly and smoothly and once an object is detected, it remains within the vision of the sensors, and a sound alarm will be emitted , louder and louder as the vehicle gets closer to the obstruction. The advantage of electro-magnetic is that they can be placed behind plastic bumpers, without drilling holes, and preserve the original bumper or fender, unscathed.