While cycling I very often find Beginner Driver tags on the road with their black-brown magnetic surfaces on top. These are apparently dropped by cars for various reasons, but once they are placed on the tarmac upside down, they become indistinguishable from the road.
The only agent that can potentially find these is the one looking at the road carefully enough to be able to notice this slight colour difference between the back of the sign and the road surface.
Cyclists, stay back and pick up these signs and sell them back to the drivers! Get them to eat their droppings infinitely!