in california (and i would assume elsewhere), once a shop takes possession of your car, they are responsible for it. EVEN if you sign a waver releasing them from responsibility. i found this case cited a lot when dealing with the laws around auto service liability since they love to claim that they are not liable (they might have signs hanging that say "not liable for blah blah blah");
The case of Gardner v. Downtown Porsche Audi, supra note 316, reached a similar result on different grounds. Gardner left his Porsche with Downtown to be repaired. He signed a standardized form that stated: "Not responsible for loss or damage to cars or to articles left in cars in case of fire, theft or any other cause beyond our control." The Porsche was stolen from Downtown's garage. Although Downtown admitted it was negligent, the repair garage argued that the disclaimer absolved it of liability for the failure to exercise due care. Citing an 1872 California statute and the Restatement of Contracts (Second), the court found that the law of California provided that persons cannot contract away their torts when the contract involves the public interest. Applying a six part public interest test established by Tunkl v. Regents of University of California, 60 Cal.2d 92, 383 P.2d 441, 32 Cal.Rptr. 33 (1963), the court, after deeming Southern California the capital of the motor vehicle, found that automobile contracts affect the public interest and standardized terms that disclaim responsibility for torts violate public policy.
edit to add
the link i got the text from