The washer bottle sensor is the simplest thing in the world. It's just two prongs submerged in the washer fluid. When the level gets low enough, the two prongs are no longer submerged and the visual/audible warning goes off.
The problem is that, as you go down a hill, all the remaining washer fluid runs to the sensor side of the tank, submerging the prongs again. As such, the cluster thinks you've refilled the tank. Then, when you get back to level ground or go up a hill, the fluid flows away from the sensor and the prongs are once again exposed, causing the warning to go off again.
It's a slightly daft design. Having an additional sensor at the top of the tank (or applying some electrical hysteresis) would have prevented this problem. However, Groot's suggestion also solves the problem. If the warning keeps going off, just use more of the washer fluid such that there's no longer enough in the tank to submerge the sensor even when going downhill.
I would think that "Timmus" may be able to turn this off ?.
Yes, I am able to turn this sensor off completely. On my own car, I've just reduced the volume of the beep for all warnings; I don't like being given a heart attack every time I need to refuel!
Cheers,
Tom