A tough one. Undoubtedly, they'll have a sign up saying they are not responsible for any theft, damage etc etc if the car is left in for servicing. Basically an exclusion clause under contract law. For it to be effective you have to have it shown prior to you accepting to leave the car with them. That way that if they are held to be in breach of contract then they can point to the fact that the exclusion clause was agreed by you. But they can't the clause to exclude themselves from basic responsibilities and to provide services that an ordinary person ordinarily would. The exclusion clause has to be reasonable.
Alternatively, if you can prove that they were negligent you may be able to take a case under tort. You can't contract your way out of negligence. So if somebody left the keys in and the doors open while they were all out at lunch then you'll have a strong negligence case.
