In Starcraft 2 - Wings of Liberty there was one hidden mission (details here), does Heart of the Swarm also contain a hidden mission, and how do you get to it?
Having completed the campaign and gone through the achievements I will say there is no secret mission in Heart of the Swarm campaign.
The game consists of 20 missions where some of them have an optional order and 7 simple evolution missions that each let you choose 2 directions to evolve a unit.
There is a secret mission accessed in the final portion of the Dr. Narud series. During the mission you will be told to avoid two areas because they are not relevant to helping Kerrigan. These two areas contain the Tal'Darim bases. Ignore the warning and find a way to clear both bases while still managing to keep Kerrigan from dying to the null ray. If you do this you will be told that the swarm has found some of Narud's personal artifacts. These will lead to a side quest with some fun Easter Eggs and a hint that suggests that
the identity of Narud is Duran. Narud us Duran backwards same person shapeshifter