>>>>>>>if the glass breakage sensor has been trigering the alarm this could be why the battery has gone flat <<<
I can't see that myself, as The internal battery is only needed if someone disconnects the Main Car battery otherise, everything is powered off the Car battery.
But The sirens are Notorious for failing, So if it is logged as a fault it will need changing or unplug it and let it work on the Horn instead.
And The Glass breakage Detector is built into the HRW, So make sure that the Rear screen hasn't gone open circuit, otherwise it will false alarm and log that fault. ( seen a few )