In my jurisdiction, a mechanic who takes a car for a joyride is committing a class A misdemeanor (unauthorized use of a vehicle in the third degree)

ref. https://codes.findlaw.com/ny/penal-law/pen-sect-165-05.html

In other jurisdictions (like, say, New Hampshire), that same case falls into the definition of theft.

http://www.gencourt.state.nh.us/rsa/html/LXII/637/637-9.htm