The pumps stir the fuel and keep water in suspension in flight but what happens at a cold layover? Does it all sink to the bottom and freeze up in one layer? What causes that layer to break up again if temperatures remain below zero? Can it just sit there? If it partly melted on the ground could you drain off the water leaving the ice and convince yourself the water was gone?
Edit: Can the water sensor detect ice in a layer or in suspension in the air?