On this page of our guide to GreedFall, you will find a walkthrough of the Sub Rosa side quest.
Where to start the quest?: After completing Sub Rosa
Talk to Admiral Cabral about the case of Alba's death. You will get + 3 reputation with thee Nauts. Go to the new team and interview them; go to the tavern and talk to Getchen. Go to the specified location to tail captain Reuben.
Follow the person from a safe distance, but not too far from the target, otherwise you will lose track of him. Once the person gets to their destination, you can rest or wait and see what he throws in the box. We should wait with the arrest and look in the boxes. Go back to Admiral and tell her everything.
Now go to the place of the secret meeting, and then take part in the fight, aiding Admiral Cabral. Defeat the enemies, and then search the body of the captain and his men.
When this is over, return to the Admiral and complete your mission. You will get +5 reputation with the Nauts, along with 2200 XP.