I saw Maggie Philbin on Tomorrow's World on TV back when Volvo introduced moisture sensors into their windscreens, so at the first splatter of rain the wipers started automatically. Philbin sprinkled some water from a bowl onto the car parked in the Studio but nothing happened. A little more sprinkling and still no wiper action. Finally she throw the bowl of water over the windscreen and announced that the system doesn't seem to be working, maybe they could come back to the car later. The story I heard was that Volvo then contacted T.W. and said something along the lines of 'FFS tell that woman the sensors only work with the ignition on !! (otherwise if the car is left out overnight in the rain you'd wake up with a flat battery).'