Have they checked the tyre tread of all the vehicles that have driven that way? It’s probably stuck in the tread of some poor fucker who currently thinks the worst thing in his life is a pebble that goes click click click at low speeds. Worst of it is, he only changed his tyres to get the ones with the environmentally friendly rubber that doesn’t kill all the fish in the rivers that cross under roads. I hope those little fish appreciate his sacrifice.