Reveille Brotherhood of Steel Quest
You will get this quest after exiting Fort Hagen at the end of Reunions quest, but only when you haven't helped Brotherhood of Steel yet.
Related points of interest
Stages of "Reveille" quest
1. Investigate the Brotherhood of Steel airship
After exiting Fort Hagen, I've observed a huge Brotherhood of Steel airship pass overhead. It appears to be heading towards the city. I should investigate this new arrival to the Commonwealth.
Go to the Boston Airport and tell the soldier that you want to join Brotherhood of Steel.
2. Proceed to Cambridge Police Station
A soldier at Boston International Airport has invited me to join the Brotherhood of Steel. In order to get started, they've told me to find Paladin Danse at the Cambridge Police Station.
Just reach the area.