Grocery cart policy

Idk if this is everywhere around Germany, but you know those carts that are unlocked and left outside the grocery stores? There's always one or two laying around, just unlocked but no coin in. I sometimes use them, if available, when I don't have the right size coin with me. What's the etiquette about those carts? Do you lock them back in? Do you use them and then leave them close to the cart-stall (but not inside, so the next customer can use them too). Are you not supposed to use them at all? I had formed an idea, but got into an argument today and would like more opinions.