That fixed it

The Volvo has been telling me for about 6 months that there is an ‘Alarm system malfunction-requires service’ Evidently it is the standby battery in the siren that cannot be replaced and needs a new siren fitting. This is buried deep in the engine compartment and takes many hours to remove and refit. The google wisdom is ignore it.
Well yesterday when the car was locked the hazard flashers started flashing after a minute and the only way to stop them was to unlock the car. According to Volvo the alarm CANNOT be disabled. However there is a fuse for Siren and Data Link.
Removing the fuse sorts the problem.
Better than leaving the car unlocked or coming back to a flat battery.