On the back says: 'Keep or Rehide' 'WPG Rocks'. Just found there is a fcebook group, artists hide them to be discovered. Not an English speaker so I am not sure if that 'keeps' means that I could bring it with me, so I rehide it to be found by another person :)
I remember this being a thing in the UK during COVID. The kids loved finding the rocks and also painting them and hiding them.