They need to think out of the box, once there's no fault logged, a skill that seems to be disappearing in modern workshops.

If they suspect the starter, I'd have the car all hooked up to test the solenoid voltage appears when the key is turned. Then sneak up on it and try it one morning. If there's no voltage, work back towards the switch.

Can't be a lot more help without a wiring diagram in front of me but it's troubleshooting on the most basic level. My avionics apprentice would be able to work it out with the diagrams. So a proper mechanic should have no worries in my opinion.

Gavin