On the “Media Blitz” mission there is a part when you control Odin in the city. When you are doing this you need to go the far bottom-right hand corner of the map.
Then, destroy the red civilian building on the little floating platform. You can get to this building by following the pathway from the small enemy expansion base that is just north from where you started.
Once you destroy the small red civilian building, there will be some “secret documents” that are revealed. Pick them up with a ground unit and then proceed to complete the mission as you normally would.
Once this is all done, then the “Piercing The Shroud” secret mission will become available and accessible to you via the Star Map.
One final note – you can’t play this secret mission after you’ve gone to Char, since you have to go to another plant. So in order to play it, you should go to the load menu, load any other game from before Char, and then do the mission!