MI manufacturing plates are issued to the manufacturers based in MI or who have a manufacturing, engineering facility or other facility in the state of MI and are primarily used for any vehicles they use for engineering purposes. (Cannot be used for vehicles issued to executives for their driving pleasure.) Engineering purposes can also be defined as test driving even by those who are not engineers and for competitive evaluation. I used to run around in some really nifty vehicles with M-plates (NEO might remember the ZR 1 I played around with) . In addition, VW of America, parent owner of Lamborghini used to have corp headquarters in Auburn Hills, MI and when they officially moved to someplace in VA, a small contingent of personnel remained in MI.
So, it could be a VW owned vehicle that is being tested by other than engineering; possibly marketing, or a high level executive playing around and saying it being tested but really being used for joy rides.
Or, it could be a vehicle purchased by one of the other auto manufactures such as GM, Ford, FCA, etc., and being evaluated.
I can remember running across some Land Rovers being tested by Ford engineering in Moab in, I believe early 2007, with M-plates. Talked to them out at Tower Arch. They appeared to be having more fun than actual testing, but by that time, it was on the selling block.