!Spoiler!
In the Mission of "Media Blitz", go to the Bottom Right corner of the map, and you'll find a Red Science Building. Destroy the building and accesses to the Secret mission will be unlocked.
Secret mission: Raynor and his raiders venture to a Dominion science platform in space, where they find Hybrid clones. With Raynor discovering this, he destroyed the platforms reactor, causing major malfunctions, and a Hybrid to get loose. Raynor had to then escape to a dominion air craft to exit that platform, and escape the loose Hybrid. As hes in the bridge with Matt, the start to wonder... Why did the dominion create these Hybrids? and how many more science stations like this one are there?