it is one of the classic Audi alarm randomly going off question. It can be anything from a dead battery inside the siren unit to a dying car battery (which was my case). Check the main battery first, it is free and easy. Then you want to get a diagnostic scan, it shows which sensor triggered the alarm. Reset the fault codes, wait for the alarm to go off and re-scan to double check which part set it off. If your siren unit's battery is dead, you'll hear 4 beeps from the back of the car after ignition.