Out in public, I generally leave my dog alone in the car for about as long as it takes me to pay at the gas pump outside and get back to the car. He typically only goes if he can come out of the car with me, or if I have someone else to wait with him for a short amount of time while I run in to do a quick errand, or if we don't have to leave the car (he thinks the drive-through is the funnest amusement park ride on earth).

I don't worry about dognapping much, b/c as soon as the excitement of being in the car wears off, he crawls under the back seat and dozes off. Unless something interesting is going on, you wouldn't even realize there was a dog in the car.