The car behind my question is a 1999 S80 2.4 but I believe the root problem affects all P2 platform Volvos.
The backup batteries in the anti-theft alarm have a finite life. When the end of their life is approached they fail to take a charge with the consequence that the main car battery becomes flat due to a continuous attempt to recharge the alarm battery when the car is parked up for an extended period.
The problem is that the alarm sounder is a sealed unit that also includes the battery. Volvo "repair" is to replace the entire unit at £££s ( or $$$s no doubt too )
Is there a known hack to take the unit apart and replace with an equivalent consumer battery, please? I don't want to buy a second hand unit and then have that fail too.