Exactly! Makes me think of things I have heard and read about historical sites getting worn down by visitors or people finding artifacts while digging in their garden.
There is probably not an easy way to do that on the spot. Do you think the artist traces out the dimensions and then makes the piece at home and brings it back?
No they just pour in the mortar and then put the pieces in. The ones that do pictures like a cat or a bird or something premake the picture part and then fill in the pattern surrounding the picture afterwards. This particular french artist does it anonymously in the middle of the night.
Here in the UK people spray paint giant dicks on the floor around the potholes. A slightly alternative way of annoying the council into taking action.