If they are older dates then possibly you have a ghost who likes to "bring" you stuff. I have several ghosts but they bring me coins inside my home or inside my car and not in the yard. It's amazing considering I use plastic to pay for everything that I keep getting a pocketbook full of coins in the bottom that I have to empty out every so many weeks or my purse is too heavy. Mine bring me every denomination of coins. My son found several dollars in coins inside his locked stand up tool chest in the bottom under the bottom drawer. Funny thing is he doesn't go out shopping to get change of any sort.