The alarm has a battery backup so will go off if armed and the battery power is disconnected.. If the battery is flat while the alarm is armed then this is essentially the same thing...
If you alarm is being tripped though this can be seen by using VCDS to check the fault codes in the central locking module... that should tell you which sensor is tripping the alarm all the time..
Could also be the battery in the alarm itself too... I think that has a habit of tripping the alarm if its on its way out...
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