Parking sensors mystery

[DigiMagic]

There are 4 distance measurement areas on the radio for the front parking sensors. And there are 4 sensors.

There are 4 measurement areas for the rear sensors too; and as far as I can tell they all work properly. Yet there are only 3 sensors at the back of the car. Are the 3 visible rear sensors just fake decoys and 4 real ones are hidden somewhere? Or if there are really just 3 sensors, wouldn't it be simpler and cheaper to just display 3 sensors on the radio?

(This has been bothering me for some time. )

 

[vc-10]

My Fabia is the same. I believe they use clever mathematics to work things out. 4 zones, each of which is based from the echos from 2 sensors. It's cheaper to use fewer sensors and do some maths than it is to fit multiple sensors. The presence of the number plate and grills on the front makes it harder to do there, hence why it has 4.

This only really works up to a certain distance, and up to a certain spacing between sensors, and it's not as accurate as actually having more sensors. But on a cheap small car it works well.