Could a faulty sensor cause an intermittent beeping on a domestic fire alarm system (professionally installed)

We had an intermittent beeping on the system which we notified the electrician of when he serviced it. We still had the beep afterwards and when he (eventually) came round said it was caused by a different fault ( new battery needed) and charged us a second time for this work. Please note it's not a basic fire alarm but a professionally installed linked unit (Niceic approved). Are we being fleeced or is this a possibility?