Just had this in mine
Assuming (as its an initale) that they are factory fitted sensors, the box and processing unit is tucked away, left hand side inside the boot (I have the tourer, but its probably in the same place in the h/back)
I too thought it was a switch either on the reverse gear, or on the reversing lights (as it is on the aftermarket kit I fitted to wifeys megane)
Oh no, why make it that easy.
Its somewhere else, but I couldn't find the activator.
My garage took it in under the warranty (aftermarket - not Ren anymore) and after 3 hours, got bored looking like I did, tested the box itself and found that was dead. So I got a nex box.
Beeps are a bit different in tone and the distance they work to is different, but they work and I haven't yet hit anything.
If you do find the activator, please advise as this is not an uncommon fault I am told.
But it could well be the (black) box hidden in the boot, left hand side, just in front of the rear light cluster