Pyronix Belle unit chirping (Veritas 8 alarm)?

We have a Veritas 8 security panel, connected apparently to an external Pyronix Belle audible unit.
Recently, the external unit has started intermittently "chirping". It's not the full alarm siren, but a chirp something like some car remotes cause, but much louder.

Does this indicate low battery or something. (I understand that the battery is constantly charged from the mains).

System was fitted around 7 years ago