Accenta G3 detectors beeping intermitently?

I have an Accenta G3 400 alarm system. All of a sudden the detector in the lounge has started beeping at roughly 30 second intervals. The alarm was fitted by the house constructon company. It has been active for 4 years. It beeps three times in succession each time. Does anyone know why and how it can be rectified?

Appreciate your help.