What it means
This isn't the full alarm - it's a distinct, occasional chirp designed to tell you the battery is running low, long before the alarm itself would fail to work in an emergency.
What to do
- Replace the battery as soon as convenient - don't disable or remove the alarm.
- Test the alarm afterwards using its test button.
- If chirping continues after a fresh battery, replace the whole unit.
Likely causes
- Battery simply due for replacement (very common, not a fault)
- Poor battery contact - worth reseating it
- Unit reaching the end of its rated life (often 10 years)
Rough UK cost
£5 - £10 for a battery; £10 - £30 for a full unit if needed.