Note: In two separate playthroughs, we verified that, if you join the Brotherhood of Steel before starting Reunions, this quest will not appear. Because of that, we've split this walkthrough into two parts.
Read the IF YOU'VE COMPLETED REUNIONS section for a quick explanation of what you need to do and where you need to go. Read THE CASE OF THE MISSING QUEST, our alternate walkthrough and explanation, to learn what could also happen.
The latter details the game as of November 2015. It feels like a bug, given that we'll show you how to earn 300 XP without doing much of anything. We wouldn't be surprised if Bethesda modifies these events with a patch.
IF YOU'VE JOINED THE BROTHERHOOD OF STEEL
This inaugural quest for The Brotherhood of Steel makes the assumption that you did not join the Brotherhood as you made your way to Diamond City during the main story progression. Progress along the main quests until you complete Reunions and emerge from Fort Hagen at the end of the mission. You'll see the Brotherhood's airship passing in the sky.
You can either head to Boston Airport at the mid-east edge of the map to meet with the Brotherhood or head straight to Cambridge Police Station, the final destination of the quest.
If you happen to save the Brotherhood at Cambridge Police Station before triggering this quest, do not join the Brotherhood if you would like this quest on your record. In this situation, complete the Reunions quest and return to the police station to complete Reveille.
THE CASE OF THE MISSING QUEST
When players leave Fort Hagen at the end of the Reunions quest, they'll see the Brotherhood of Steel airship fly over on its way to the Boston Airport. During that animation, a quest will open up called Reveille.
The text of the quest is pretty bland, asking you the investigate the airship. But, depending on whether or not you've already encountered Brotherhood units in the Commonwealth, that quest may be bugged. Here's how it shakes out.
If this is your first time seeing the Brotherhood in Fallout 4, when you make Reveille, your active quest will show a quest marker at Boston Airport, where the airship is moored. If you're in power armor, remember that you can simply walk along the bottom of the bay with very little radiation damage.
When you arrive at the airport, the brother Knight at the gate of the Brotherhood's forward operating base will direct you to the Cambridge Police Station, where you'll find a small Brotherhood unit under attack by ghouls.
However, if on your way from Sanctuary to Diamond City earlier in the game you happened to run across the shootout at the Cambridge Police Station and helped the Brotherhood unit there clear out the ghouls, but you didn't elect to join the Brotherhood at that time, then you may have a problem. The Reveille quest marker won't appear on Boston Airport, and when you show up, the brother Knight there won't have any dialogue options for you.
Reveille will essentially become a dead end quest with no information on how to complete it.
If this happens to you, simply return to Cambridge Police Station (fast travel works well), and you'll immediately complete the Reveille quest. No conversations, no combat. Just show up, and receive your 300 experience points.
Note that we're still not sure yet what happens if you make it to Cambridge Police Station, assist the Brotherhood unit and then join the Brotherhood before the airship flies overhead.