Wow - you learned the hard way. These days, you need to ALWAYS lock your car, not only when you get out, but when you get IN. That's the first thing I do, even in broad daylight. Carjackings have become a common occurrence, and not just in large cities :-(
I was driving a car to AZ, for a friend, and it had no 'beeper', like my Lincoln. I am so used to hitting the lock button, while walking away, and remembered, but kept going anyway (really had too much coffee)! Duh! Hold my beer, would ya'!
I can't plead ignorance, just temporary stupidity!