Well, then you get into the issues of sub-bailment. Is the shop the same enterprise as the avionics shop? Is it a separate entity? If separate, and the property was put in the custody of the mechanic shop without the owners knowledge or consent and the avionics shop authorized the work, then the avionics shop may well be liable for a portion of the mechanics bill to the extent that they stood in the owners shoes and had the responsibility to act as a prudent aircraft owner and manage the services provided. The mechanic still has the right to maintain possession of the aircraft until they are paid,by one or both parties, and the owner isn't entitled to a free ride and unjust enrichment because they ultimately gain the benefit of the services. This is of course assuming there was nothing nefarious or criminal going on here. If one of the party has dirty hands, different story. In your garage example, the garage owner would be liable for the wash unless the owner was aware of the service, and would be liable for any damage the washer may have caused, but the washer still has a security interest in getting paid for their services, and ultimately the bailor got the benefit. Sub bailments get complicated.