This affects all 7th Generation saloon and Tourer when the battery (The battery in the engine bay not your key fob) isn't holding a good charge. When the vehicle is unlocked with the key fob the siren sounds continuously and cannot be stopped. This is normal when the battery has low voltage or is dead, when you unlock the car with your fob , it sends the signal to the MICU(Multiplex Integrated Control System ) to unarm the car,but when it senses low voltage from the battery it goes into a loop.Due to this low battery voltage the MICU is unable to communicate with the Alarm Control Siren(ACS) to signal it to disarm it. Because the MICU and ACS now have different armed/unarmed status the siren will sound using its backup battery and will continue so long as the MICU and ACS remain in different armed status. The Fix You need to charge that battery up to full charge. Then lock the vehicle with the key fob.Wait, approximately 15 seconds, for communication to be established between the MICU and ACS. The indicators will flash rapidly to indicate when you see this both MICU and ACS are in sync Unlock the vehicle with the fob the problem is solved.