The Backup in a Box has an alarm that will sound if it believes that the mains water has been running for more than 4 hours through the unit.
If you have been for example running a hosepipe or watering the garden for many hours and your tank was low when you started then this will explain why the alarm is sounding.
The water only flows into the tank at a rate of 4 litres per minutes so if you are irrigating the garden you are more than likely using more water per minute than what is going into the tank. This would explain the alarm.
Switch off the unit and restart and then you will have another 4 hours before the alarm sounds again though you will probably run out of water in the tank before this happens if irrigating.
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