So there's a Brotherhood Initiate I found on top of Boston Public Library, near what I assume is a Vertibird landing platform (I've not encountered a Vertibird yet) . I am wondering if there's a reason for him being there related to a quest or story. It seemed odd to me, given that he is the first Brotherhood initiate I have found. My game isn't progressed very far; I am 2/5 of the way through Act 1, and have not joined the BoS or ever seen a vertibird. Googling didn't bring up any info.

random encounter?

I'll have to check in my game and see if there is an initiate. On top of the library? hmmm I didn't even know you could get on the roof.

Well, Fallout 4. Every npc in the game just love to chill out in a roof. Like Daredevil.