I think the problem will become reality very soon. Two explain the anti-theft solution, I will divide cars into two categories - private and shared driverless cars.
For private use, I propose the car "key" and a fingerprint, password, or something you defined as a login option (like on a smartphone).
In shared use, I propose two-way authentication with a mobile phone, email, or "token-like device" when unlocking the car. The authentification method should clearly identify you. That way, you would get a single-use key, and only after confirmation by entering the key in the car system, you would be able to pick a route and start your trip. The person being driven would be clearly identified and the possibility of theft would be greatly reduced.
If not successfully authenticated, car would remain blocked.