After you kill Northy, or more accurately, he seems to kill himself, you're supposed to check the body for clues. With the Silver Shroud missions you have to toss a calling card onto the character's body after you kill them to register as complete."Search body for clues" may not check off after searching Northy's body leaving the player character with a quest blip over his body. After I killed Northy I checked his body for clues and listened to the tape. Using console commands can skip the dialogue, too. Use VATS to take him out. Be sure to leave calling cards on both of their corpses. There's a blue door with a Nuka Cola machine on the left and an "open" sign on the right. Talk to Sawyer behind the counter and ask for work. Talk to Sawyer behind the counter and ask for work.