Washing the car this morning, I felt something scrape under the sponge. Inspecting the area revealed several hard black objects sticking to the body. The objects were hard to see because the car is dark grey. They are about 1/8 in. (3 mm) in diameter; they crumble easily; and they are magnetic. There were only about 6 of them, but each one left tiny magnetic flakes behind that were difficult to remove from the car.
The first one left a minor scratch which, thankfully, buffed out. Since I was aware of them, the rest were removed before they could cause any damage.
What are they? They seem to be some sort of magnetic road debris. I have never seen anything like this on any of my cars before.