Hey guys, I'd hate to disrupt "hypothetical situation hour"... i'm surprised no one has mentioned a toddler throwing the keys out the window while on the highway... but anyway, I digress.
What happens when you press the start/stop button while you're driving? It's something I didn't see in the manual and I always imagined that there would be a safety switch that didn't allow it to turn off? After all, it's a giant button right next to the wheel and prone to bumps. I'm not brave enough to try it myself, but this article got me wondering...
http://money.cnn.com/2015/01/06/autos/ford-push-button-ignition-recall/index.html
edit to add - if you leave your car running with conventional keys in the ignition not only will your car get stolen, but now your house can be more easily broken into. I don't see how walking away from it while it's running is any different with the push button or conventional key. If you're concerned about theft, don't leave your car running, take the battery in the house and secure it in a vault, and finally weld the doors shut. That'll stop 'em.