Bucky's happiness makes me wring my hands for him because it's rooted in so much trashy misunderstanding. Oh good! I figured part 4 was my chance to have the reveal happen because Bucky was unhappy, that is, to have him experience distress to a point where he can't keep it together and lets something slip, tipping off Sam and Steve. Since I let that one go (we almost got there, but not quite), I thought I should take it full throttle the other way, and have the reveal happen because Bucky was so happy and relieved (due, admittedly, to his trashy misconceptions) that he lets his guard down.