I don't leave my bike outside on a car rack unless I can see it, and even when I can see it I usually use a thin cable just to keep someone from casually walking off with.

I don't see any roof racks with enough security to leave my bike outside in plain view with me gone for a while.

My dad (I'm in my 30's lol) has an SUV with tinted windows, he leaves bikes inside the car while eating and he can't see the bikes and it hasn't been a problem. But that doesn't advertise that there are bikes in the car.