Perhaps a trail marker or a calling card for someone to say "I was here." For a while in my town it was a big craze for people to paint rocks and leave them everywhere. Some even had inspirational messages. My daughters girl scout troop even painted bunches of them and took them to parks and wooded areas, I thought it was a nice thing until whenever we went to parks or other towns all my kids wanted to do was look for painted rocks. There were also rules, you were just suppose to find them. If you wanted to keep it you had leave another painted rock. I still have painted rocks in my gardens from the ones my kids collected.