What do you mean by "going into fault"?
My RC parking sensors often beep when drawing to a stop at lights in cooler weather or in rain. Presumably as the water runs down the bonnet etc., over the sensor. In cooler weather it tends to be the fronts that sound, again presumably as the warmer air from under the bonnet affects them by distorting the air in front of them. I had this a lot in my AMG as the 5.5ltr pumped out a lot of heat!