Intellisense Securite 800L+ intermittant beeping-chirping?

we are in an estate of 18 houses, built 2half yrs ago. all with the same sytem. two weeks ago two outside alarms started to chirp/beep at 5.30am at minus 3 and the chirping stopped at 10am at +2C.
This week another 4 alarms have all started chirping at 1:30ish with air temp of minus 3-6C.
entering keypad codes etc. makes no difference.
We have disconnected outside battery and 12+ supply to the box to silence it.
Is the -C temp significant? As all batteries have good voltage when tested and none of the other house alarms chirp.
What could cause the chirping to start at night not during the day?
None of us has a service contract, so could it be a service counter? Only a few have used their alarms.
The outside box is a Gardiner Technology system GA series PR5508!