I like to be prepared for such eventualities. I'd recommend getting another spare key that will unlock the door and attaching a piece of wire. Grovel round under the car until you find a good hiding place. Inside the body - behind a rubber plug means that your key doesn't see any mud. Otherwise wrap it up in insulation tape and tie it onto something with the wire. I just leave a little bit of wire hanging down to make it easier to find and extract. I also leave a spare ignition key hidden inside the car. The chances of someone finding both keys and making off with my car are pretty remote. The chances of anyone wanting to are probably equally remote!