I was told that you have a duty of care as a professional to discharge any knowledge you have obtained regarding a vehicle to the customer/owner. We have no legal right to hold a vehicle if the customer has paid the bill. In fact, if a customer takes a vehicle without paying you can't even impound it or retrieve it for security, it's classed as a civil matter, not a criminal one.
Maybe things have changed since I was told all this ( 8 or years ago )