To tune in an engineering sense, is to make fine adjustments e.g. you tune a car engine to give an optimal performance. The water film value of a road will vary and a road sensor needs to be adjusted to detect minute levels of water on the surface.
My understanding would be that non-tuned water film is surface water which is not detected by the sensor i.e. below their levels of sensitivity.
But I´m not an engineer!
Does that help?