I see three possible sources for the failure:
2.: "Sensor" itself
3.: Faulty electronics
I have painfull experience with this problem too. In my case it was "2.": just clipped off the "sensor" (just two pieces of metal that, for some unknown reason, react very sensitive to moisture and gave routinely false alarm). Now my "sensor" is just the two endings of the clipped copper-wire: also reacts to moisture but much less sensitive...