they can drive off but the car really wont go far with out the key which is mostly in my pocket, did a small test and the car only goes about 15mtrs and slows down and stops.
I dunno about the A5 in the states, but my A5 delivered from Australia isnt very safe at all. We actually did that test. I was about 10mtrs away from the car, my mate jumped in, started it and drove away. I was under the assumption that the car would pull itself up a short distance from where i was standing, though to my astonishment the car kept going and going until it disappeared around the corner!!! I dunno if there is a difference between here and the States but i was very concerned when the car didnt pull up at all!!!