Just a quick PSA. Earlier while finishing the Curtain Call quest at Trinity Tower, I had MacCready with me as a companion. He did not follow me down in the lift with Rex and Strong and I dismissed him to Sanctuary when prompted at the end of the quest in order to take Strong as my companion instead. When I got to Sanctuary, he was nowhere to be found. Ringing the bell did not call him and he was not in Goodneighbor either.

Going back to Trinity Tower, I searched every floor with no luck. Deciding to take the Rex/Strong lift down again, I noticed MacCready was standing in one of the rooms that the Super Mutants spawn in during your escape down the tower. The only way into that room is to jump off the lift while it descends and then fast travelling out after. After talking to him here, I was fortunately able to get him to join me again. Without the lift however, MacCready would've been trapped in that room.