Recently, my lady took off with my key, leaving me stranded. For next time, I put the extra remote in the trunk next to the tire inflator, but the car kept telling me I left the remote in the car. So I put the extra remote in an Altoids tin. That attenuates the signal enough so the car doesn't complain. So next time I lock myself out, I can unlock the car with OnStar, and start the car with the hidden remote. (Or a spare metal key I also zip-tied to the undercarriage.)