The achievement is unlocked when you view Chairman Rockwell’s video message. It is story related and cannot be missed. Nevertheless, how you unlock it depends on which branch of the story line you decided to pursue during the game. If you chose the Halcyon Path where you notified the Board you knew Phineas’s whereabouts, the achievement is unlocked during the main story quest Signal Point in Space. If you chose the Phineas Path where you did not discuss Phineas with the Board or explicitly told them you did not know where he was, you unlock the achievement during The City and the Stars main story quest.
On Groundbreaker, if at any time you tell the Halcyon Ambassador Udom Bedford that you know where Phineas is, you embark on the Halcyon Path. This branch takes you to Byzantium where you interact with Adjutant Sophia Akande. During Signal Point in Space, you must deal with radio broadcasts on Monarch. Once you have completed this tasking, you report your success to Sophia. At this point, she automatically shows you Chairman Rockwell’s video and the achievement pops.
Conversely, if you keep Udom Bedford in the dark with regards to Phineas, you embark on the Phineas Path. You go first to Monarch and complete the main story quest Radio Free Monarch. Afterwards, Phineas asks you to obtain chemicals from Byzantium. Ostensibly, Minister Clarke is responsible for their storage and dispensing. However, it turns out Minister Clarke is not a free agent but is under house arrest at the direction of Chairman Rockwell. To gain access to where the chemicals can be obtained, you must print out a keycard from Rockwell’s terminal. When you access the terminal, you view a video message Rockwell recorded and the achievement pops.