Here in the Upper Midwest, one of the rites of spring is to hook up the garden hose and wash away all the crap that has built up on the floor under the car during the winter months. When I got ready to do so last week, I was startled to see four or five yellow spots. The embed above may not do the color intensity justice; they weren't just yellow - they seemed to be of neon brightness.
I immediately popped the hood of my car to check the brake fluid, and to my delight and surprise, the reservoir was totally full. The position of the spots was midline, not near the debris from the tires and wheelwells, and not near the axles.
So I asked Mr. Google, and discovered that these stains result when road salt gets up onto the hot components of the exhaust system. Those components often are coated with zinc as an antioxidant, and the road salt can catalyze a reaction resulting in the formation of a yellow zinc oxide, which is most often spotted in the snow under cars parked outdoors.
I had my Subaru in today for its 11-year checkup, and the staff confirmed that the exhaust system shows no evidence of corrosion. So this was just an incidental finding. Good to know.