you need to accept the quest: Eradicate Sub-Guh'Nir given by Galiano. Day must be 7 or more, you must have discovered the core door in the subway facilities and Sammuel must be part of your crew.
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Ah, alrighty, thanks a lot for the hint.
I think when the Game reaches a certain complexity in the story, it would be nice to have a Wiki or at least "Walk-through" at some point to be able to see, if all the content/paths/possibilities have been explored. Might also give players a nice inspiration to re-play it maybe (unless they play through with the Walkthrough next to them, of course).