If he is used to the car and other dogs, its a possibility that he is lovingly owned but escaped. You should post flyers up, contact the Humane Society and check to see if anybody has come in looking for a lost dog...

Go to the vets to see if he is microchipped as well. Somebody could love this dog very much. What condition is he in? What breed(s) do you think he is? Better get some pics too...is he wearing a collar?