This quest will be available in one of three ways:
If you come across the fallen members of the Brotherhood recon team in the east and northeast region of the map before meeting the Brotherhood in the police station. Then speak to Paladin Danse or Captain Kells.
If you complete the Show No Mercy Brotherhood quest. Then speak to Captain Kells.
The first three steps in this quest is to track down three fallen recon team members. By way of the waypoint indicator, head northeast until you reach Med-Tek research or Malden Middle School. You should be able to detect a distress signal. Tune your radio and move south to increase the signal percentage to 100%. The remains of Knight Varham will be in a roofless house slightly south of the research building. Pick up his holotag and holotape and listen to the tape.
Your next stop will be southeast to the National Guard Training Yard. Once again, use the radio to increase the distress signal. Expect some minor confrontations against several Ghouls. You'll find the remains of Knight Astlin in a second floor room. Pick up her holotag and holotape and listen to the tape.
Head east once again, this time to the Revere Satellite Array. It's hard to miss when you come out of the training yard; look for the large dishes. Firefights here will be more intense this time, given the strength of the Super Mutants. Look for a shack hanging off the edge of one of the dish towers. There you'll find Scribe Faris. Pick up his holotag and holotape and listen to the tape. You'll also receive Recon Bunker Access Code 429A.
This will lead you to Recon Bunker Theta at the far north edge of the map. Use the access code to open the bunker door. This will trigger a standoff against the lone survivor of the recon team, Paladin Brandis. You can choose to kill him or end the confrontation peacefully. To accomplish the latter, reply to his questions with non-threatening responses. End the conversation by handing him his team's holotags. Speak to either Paladin Danse or Captain Kells, depending on how far you are in the main quest.