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Alarm functions are incorporated in the central locking control module.
The central locking control module should be interrogated to determine the cause of the alarm triggering. Every trigger event should be recorded in the fault memory.
About the only thing you can do as an owner without a Audi/VW capable scanner would be first to disable the interior monitoring system. To do so after turning off the engine and opening the door, press the button on the "B" pillar (your seat belt is mounted in the "B" pillar) down near the bottom. There should be two buttons, one for the trunk opening, the other to disable the interior monitoring. Once you open the door, press the button, the little light inside the button should illuminate. Close the door and lock it. The alarm should now be set, but without the interior monitoring. If the condition no longer exists, there is a problem with the interior monitoring.
If this does NOT correct the issue you should assure all the door and hood/trunk contact switches are functioning.
For the doors check the interior lights are coming on and off with each door and not illuminating if you push out on any door (without unlatching) while in the car. Also if you lock the car with a door open, then close the door after, the alarm should give a single beep indicating the contact has closed. This won't work with the driver's door as it will not lock with the door open.
The same method will however work with the hood and trunk. You can also check the trunk switch by having someone close the trunk with you in it (possibly better to have your wife or husband inside), and push upwards to see that the illumination does not come on unless opened.
This year/model is also susceptible to water intrusion into the Central Locking / Anti-theft control module. I recommend you go into the trunk remove the panels or trim to access the right rear corner below the right tail light assembly. There you should find the control module. Inspect for water and or corrosion on the electrical connecters. Typically what happens is the seal at the taillight fails (previous collision?) and water gets in. If this is the case you may have luck taking the controller apart, drying it, and cleaning the electrical contacts. If it has been like this for awhile you may need to replace the unit. Be sure and water test the trunk before returning the vehicle to service.
If all tests good, I suggest you take the vehicle to an Audi/VW specialist who has a scanner capable of interrogating the central locking control module and checking for and retrieving any (likely many) stored Diagnostic Trouble Codes (DTCs). The DTCs will indicated the circuit that has triggered the alarm in all instances of alarm activation.
I hope I have answered your questions and addressed your concerns. Should require any further assistance on the matter, please do not hesitate to ask. I want you to be 100% satisfied with my answer.
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Jake "The Audi Doctor"