Ring! "Hello?" Car: "It's your car." You: "My what?" Car: "I've just been stolen and have alerted the authorities." Okay, that's a call nobody wants, but GM has just added a feature to its OnStar security system that does exactly that, called "theft alarm notification." It works with appropriately equipped OnStar vehicles to contact you via text, email or phone if a bad guy activates the alarm on your GM vehicle. The feature is an add-on to GM's existing security system, which already lets OnStar advisers contact the police, track your vehicle and even slow it down or shut it off if it's safe to do so.