The next section of the Greedfall Guide contains a walkthrough of the side mission The Spy from Theleme.
Where to start the quest? After the mission the Mrs. Admiral's Intelligence.
Start exploring the port of San-Matheus. Ask the for Alba, and you'll learn you need to talk to Alba's father at the tavern. The person only visits the place at night. Talk to a man about Alba, and you will be tasked to find a spy Ordo Luminis.
Go to Wenshaganaw. "Singing Waters" and eavesdrop on the men. Then talk to Johannes - you can bribe him with 200 gold, or try charisma. In the end, go to the captured Islander and release him.
For the assignment, you will receive 1400 XP, +3 reputation with Nauts and begin the mission Sub Rosa.