I have always tried to get a spare electronic key when it was not needed. There are lots of locksmiths that make them up for $50-$75. A good suggestion is to have another person hold your spare key, or keep each key in a separate spot.
I bring a rental car to the playa, and if I get only one electronic fob, I make a copy of the hard key to open the door and lock the electronic fob inside the car. If there are two fobs on the key chain, I break them apart and keep them in different spots. One being tie wrapped in my tent or camper.
I tend to be a bit obsessive with these small details. A guy camping with us this past year had only one key to his car, and he put it in his back pocket of his pants---which had a gaping rip in it. His keys were turned in toward the end of the week, but he spent most of his time trying to figure out how to get a tow truck out there and what dealership to bring it to so a new key could be made.
Needless to say, leaving your backpack unattended was a mistake. Someone seeing it sitting there could have thought it was dropped and picked it up to turn it in. Second, there should be some sort of ID or address information in it, so it could be mailed back to you. This wouldn't help you get your car started, but at least you might get your belongings back.