With the water alarm, if you use foam, and it fills the tank (Which can some times happen), foam gets up the water level dip stick. (Small tube, runs from top to bottom, in the tank, with little metal strips which if in contact with water, (conducting electricity) show the little lights as full, 3/4 etc.) Now, if foam gets up the dip tube, or dirt, or silt from the water etc, it can some times stick too, or clog up the dipstick, thus stopping water from getiing in, and thus making the truck think there is no water.
Have heard of it happening to a few appliances, especially after using foam, or using extremely dirty water.