The intelligence gathered from the Outside The Wire quest led to an unfortunate discovery: A loyal member of the Brotherhood, Paladin Danse, is a Synth. Worse yet, he didn't even know he was one until this revelation. This quest, initially triggered by Elder Maxson, will lead you to Proctor Quinlan. The ensuing conversation will trigger the appearance of Danse loyalist Scribe Haylen. Speak to her.
The journey to confronting Paladin Danse will lead you to Post Bravo in the northern Commonwealth. Deal with the defenses however you like, then go inside the small base to meet with Danse. You can use your powers of persuasion to prevent Danse from being an example for all other Synth, or your lack of charm will lead to an untimely death for Danse. Whatever the result, you will end up with Danse's holotags. Return to the Prydwen to conclude this quest with Elder Maxson.