This mission unlocks after you take over the outpost Cradle View.
Enter the house marked with a blue exclamation mark. Inside, listen to Rebecca's story. The woman suspects her husband of cheating on her. Exit her house and approach the house opposite (don't get too close) (the screenshot).
Walk over to the right of the house and hide among plants. Approach the window close enough to eavesdrop on the conversation. Don't get too close however, or else you will be spotted. After you listen to the conversation, return to the wife and inform her of the results of your investigation. This will conclude the mission.