My wife had the same problem with her Astra Coupe.
Has this been happening in the middle of the night or on hot days?
The problem as I understand it is that there is a battery in the alarm unit itself and it is charged by the cars main battery.
The alarm system itself senses voltage change as an intrusion and triggers the alarm. With time the battery deteriates in the alarm unit and struggles to hold its charge. The instability of the charge in the battery is exaggerated when it is cold at night or hot outside causing the alarm to trigger.
whole new alarm sounder unit needs replacing I think. As I recall it cost about £150 for the astra from a dealership.