It could be hardware failure but make sure the senor’s internal battery contacts are clean and that the battery is new, the correct type recommended by the manufacturer, within it’s printed date code, and following the correct polarity.
Based on the troubleshooting done Its likely hardware failure and should be removed from programming and replaced. How old is the sensor?
You can try removing it from programming, waiting a few minutes then reprogramming it to check if the low battery returns. You would want to program the sensor as a Sensor Type 23) No Response Type to avoid any potential false alarms.