I never leave anything valuable in the backseat of my car ever. And never leave any kind of bag, even if it has nothing valuable in it, it's still an invitation to steal it and find out what's in it. Sometimes I may leave a bag in my trunk when running into a store to do a quick errand, but mostly I just take the bag in with me. Whether it has my laptop in it, or my camera gear. That way it's foolproof.