Dude I honestly feel you bro!
Fortunately for me, I am like best friends with my BMW Service technician at my dealership and I wouldn't care if he drove my M5 a hundred miles because he's a professional track instructor also.
But in cases like yours where you take it to some paint repair shop, I'd be pissed too. Unfortunately we have to accept some degree of risk when we take our baby's to places like these.
Here are some things that you can do to help mitigate it for next time.
1. Show the manager that you've taken a video of the odometer, date, and time.
2. Draft up a legal statement, stating that the workers of the repair shop are not to drive the car any more than necessary. Then have the owner or manager of the shop sign it, and tell him that you will take this to court if it is violated.
3. Intall some sort of video camera that is pointed towards the drivers seat so that whenever the car is started it'll start automatically recording. Like those dashcams that people use all the time, except have it on the interior and pointed at the drivers seat. Some companies do this to their business cars too, to make sure that employees aren't out there F'ing around in them.
4. If extra miles do occur, inform the manager, that you will go on every social media and customer review website and give them bad reviews. This will damage their business.
Well that's all I can think of bro. If all else fails, you could always find out where the manager lives and then proceed to egg his house and car. Just a thought!
