Document everything that was said to you by their GM, Tech, and staff people. I'm interested to see if you were lied to about this supposed "random test drive". Sounds like a load of bologna to me. If they were joyriding your car for no reason after all work was finished then you could sue them. You need to expressly authorize all work before it is done.
2005 330i | ZHP, Xenons, Jet Black/Natural Brown (Black Cube)