The gray area is that the AG opinion on OC from 1930ish was that the firearm and the holster must be in plain view...
With this being said, there was a newer AG opinion that said that as long as the firearm is discernible by the average citizen then it is OC.
So, as it sits, carrying a firearm in an IWB holster seems to be legal according to AG opinions, it does leave a window for Wayne County Prosecutors to harass you and make you spend a @#$% load of money due to the fact that there is no CASE LAW to set precedence to the matter.
However, This also brings up another very important question. The older military style flap holster COMPLETELY covers the firearm, thus the firearm is no longer discernible by the average citizen.
But I can safely say that in WAYNE COUNTY, if you are a non CPL holder, THEY will come after you for concealing your holster even though the firearm itself is completely in the open.