..... (_!_) Sarah's A3s #1 Fan
Don't know where you are or if you have a LHD, but pipe on (Avant) A6 runs on the passenger side as does the Convenience Module (CM). Mine popped off with the big freeze and filled the black platic box, which holds the CM and gave me all sorts of electrical issues, electric windows going down when unlocking car, interior lights flashing on and off, central locking to all doors popping up and down etc..
However, someone had the alarm issue you describe and it turned out his Mrs had used the car and hadn't closed the tailgate properly.
So, could be the tailgate or one of the doors?? You will need to pull back the carpet to get the CM out and see if there is water present, if so, it needs to be dried out and all contacts cleaned and checked...
Let us know how it works out.