OK , I was a mechanic at a dealership and often we would have the local P.D. bring their cars in to be maintained or repaired. The mechanics were allowed to test drive just as any other car. A mechanic would never take ANY vehicle home...none the less a police car.
As far as "What if someone flaged them down" goes well seems sort of easy to just say "Sorry, Im a mechanic and im test driving this patrol car" and that they should call the Police. very simple.
"Should they sign on to the MDT?" Of course not, like Stump said what are they gonna sign on with?
One of the main rules of working on Police cars was pretty much not to touch ANY of the emergency type equipment while test driving the car....pretty much all of that type of stuff can be repaired and checked within the shop area anyway.
"Guns don't kill people, People kill people"