It can be found in Listening Post Bravo during the quest Blind Betrayal after Danse has committed suicide. Easiest way to ensure this is to leave Danse mid-conversation, go back up the elevator and walk towards the door until you get the notification to collect his dog tags. [verified]
My identity as Paladin Danse is nothing but a memory now. Everything I held dear, everything I've ever believed in is completely gone. I've spent far too long wondering why this happened to me, but the truth is, it doesn't matter. I am a synth... which means I am freak of nature, a perversion of science and an example of where mankind has gone wrong. For the benefit of humanity, I need to die. Not because I'm cowardly or despondent, but because it's the human thing to do. This is Danse, former Paladin of the Brotherhood of Steel, signing off.