I had this fault on mine, kept me awake all night! Problem was the boot lock sensor connection had been eroded and telling the car the boot is open when it was shut, hence alarm kept goin off. This was down to poor audi build on water channels! Strip your boot trim and it will probably be the same fault. I had to change the boot lock and not the wiring luckily.
did you do a fix/clen up or replace lock?........hope lock not tooooooooo dear!Had exactly the same with the mrs A3, the plug that goes to the microswitch was soaking wet, this is the switch which tells the car if the boots open or shut!