It could be like the secret cow level in Diablo 2, requiring the player to complete some obscure task for it to spawn. Like finding some loot, that is only dropped by a certain civilian in a certain mission in a mapblock, that is inaccessible to enemies. Researching said loot gives the player a manufacturing project requiring all kinds of weird ingredients.
The resulting item, once researched, finally spawns the secret sniper chicken mission, where you have to save some Ayyys, who are stranded in the dreamscape and set upon by avian assassins.
Or something like that
