Typically the batteries on these devices don't last very long unless it's hooked to the car battery. There's zero reason for the mechanic to go through the property of a client. I'd sue the hell out of that company for doing that. It would be similar to an employee at a computer store going through your computer looking for racy photos. That's not their job. Disconnecting the camera would make me never bring my car to that place again as I wouldn't trust the mechanic.