Never heard of a shop being liable for damages to any vehicles parked on the lot. Most places have signs posted and disclaimers to sign. If he locked your car and secured the keys, his responsibility ends there IMO. If it were my car, I'd be pissed that it happened, but not at my mechanic. Nothing he could do about it past locking it up right. We always have the option of not leaving our vehicles out of our care. I can't imagine why'd he'd be responsible.