I arrived at a mountain airstrip many years ago, to do an inspection on a commercial sailplane. It wasn't complete at the end of the day. I put the various parts inside the cockpit under the canopy, closed it up, and flew away, to return the next day. When I arrived the following day, the sailplane was missing. Someone said the owner was giving rides in it. A lot of parts had been removed as part of the inspection, including the cockpit, with everything set inside the cockpit. The owner simply sat the passengers in the cockpit, nothing actually attached to the aircraft, and went flying. When he landed, he saw the look on my face, and said, "you're going to laugh when you see what I did."
I didn't laugh.
Someone takes an airplane when the maintenance isn't done (and it's not done if it's not singed off), it's all on them. That's the nature of being pilot in command. The buck really does stop in that cockpit.
A mechanic can't return an aircraft to service. A mechanic can only approve an aircraft for return to service. The pilot returns it to service when he or she flies it. It's on the pilot.