Where can I find Paladin Danse after dismissing him to Boston Airport?

During my playthrough, when I sent Paladin Danse to Boston Airport he ended up behind a quest locked door that isn't opened until the beginning of the Brotherhood of Steel mission Liberty Reprimed. Unless you're willing/able to use console commands, he can't be reacquired until you start the mission and the door opens.

Obviously, the preventative workaround is to never send him to Boston Airport - use the Prydwyn (or any other location, really), instead.

After dismissing someone to the Boston Airport, they won't be in the Prydwen or even in the BOS controlled area. Fast travel to the Airport, turn right, and go through the hole in the wall to the waiting area and They should be there.