The next section of the Greedfall Guide contains a walkthrough of the side mission The Sub Rosa.
Where to start the quest?: Complete the side quest The Spy from Theleme.
Go talk to the guide in Wenshaveye. You will learn about the existence of the caves - go there. If don't beat the guard, be careful - there's a pretty strong miniboss in the area. If you want, you can sneak into the cave.
Inside, you will find a few beasts - start tracing Alba. After some time, you will find the corpse of the woman - you will lose -2 reputation with the Nauts (you can get it back in the next quest) - search the body of Alba.
You will complete Sub Rosa. You will get 1900 XP and automatically start The Enemy Within.