C6 alarm chirruping

[PaulC5]

Our C5 battery does not hold its charge if the car is not used so I thought the alarm might be the culprit. I disconnected the siren but this did not help, just put the alarm red light on. Disconnecting the control unit did not help either but put on the hazard lights when the engine was running so I had to connect it back. They are still the original parts at 17 years old so might not be at their best. On the C5 the control unit is in the left hand front door pillar area and a bit of a pig to get out without removing too much of the trim. The siren is in the left hand side of the boot behind the trim but needed the bumper releasing a bit since it got in the way of moving the siren. Using a meter on the disconnected siren unit does not indicate any voltage but that might not mean anything about its battery.