Painted Rocks?

An old high school friend of mine, who does not blog, but should, has gotten into painting rocks.  It is just what it sound, she paints designs or decorations onto small stones and then leaves them in public places or gives them away.  She lives in Florida, I asked where she finds rocks to paint, and her answer was aisle 3 at Lowes. The goal is to leave a little art, in a public place, to add a little charm and adventure to the day of the person who finds it.  
  
I was thinking this is a local thing in Florida, well apparently it isn't, I ran across this one on the gate to the lower garden at Mt Vernon.  

Have you ever left something in a public place - hoping someone would find it and be pleased by it?