How do the interior alarm sensors work?

My car is a 2012 facelift FR.

It's been left locked on the driveway all afternoon with the windows open (was locked with the fob by pressing the lock button once so deadlocks still on and alarm sensors supposedly on).

Before I got into the car I decided to check out the alarm sensor operation by waving my hand about through the open window expecting the alarm to be triggered. Diddly squat happened

Are my sensors not working or do they get automatically disabled because the windows are open? Does anyone else's alarm go off if they wave their hand through the open window of their locked car?

Nope, only pressed the lock button on the key fob once.

I've just sat in the car with all the windows up and sunroof closed. I then locked it with the fob (one press), waited 30 seconds until the red LED on the door started flashing to indicate a fully armed status and then frantically waved my arms about. No alarm siren went off, nothing

Does the Mk2 facelift actually have interior sensors? If so, where are they located?

OK, thanks for that. I'll try and take a look at the weekend to see if there are actually any sensors behind that slotted panel. The handbook does clearly state that the volumetric sensors are activated when the car is locked which mine clearly aren't.

The car is an ex-demonstrator and the only previous owner was the garage so I wonder if they disabled the sensors permanently with VAG-COM so they could leave the car on the forecourt with the windows open and the sensors disabled but the car still deadlocked (which gets released if you double-press the lock button on the key fob if you want to temporarily disable the sensors).

It would be a bit of an inconvenience but the car is still under warranty so in the worst case scenario I can always take it to a dealer and get them to sort the problem out.

I've just been out and pulled the slotted panel out which was a simple enough job and it does contain an electronic unit with 3 volumetric sensors underneath so the kit is all there and plugged in but just not working it seems.

I might ring the dealer up that I bought the car from and see if they will own up to having disabled the sensors via VAG-COM. At least then I might have some idea if it's just a configuration change or a faulty sensor unit. They're too far away though so any fix would have to be done by one of my local dealers.

Well I've just been out and unplugged the connector from the volumetric sensor unit then plugged it back in again and the sensors now appear to be working fine. Locking myself inside the car with all the windows closed and then waving my arms around now sets off the alarm.

I'll keep an eye on it but hopefully it was just down to a bad connection with the volumetric sensor unit.