Painted rocks

Our local primary school started a pebble snake during the first locldown. Inviting the kids to paint a stone and add it to the back of the snake on their daily exercise walk. They carried it on through the summer and it got quite long. I think some other communities started doing similar things, I know one other local area was doing a kind of swap scheme where you could swap one you've done for one you like. It may be something similar.
I also remember seeing some hiding in local parks with numbers and the words "please leave me" or similar on the backs. I think they were being used for some sort of treasure hunt.