Their touch
Almost touching, ever so close but rarely ever doing it. She is surprised about the amount of self control the both of them have upon the subject. She knows he's trained to keep the personal stuff to himself and that life taught him the same early on. But she never had to act on that control, at least not for a prolonged amount of time.
So when they touch he momentarily stiffens, and then something flashes in his eyes. Longing. But it passes as quickly as it appeared and she's never really sure if it was really there or if she imagined it. But when he relaxes into their touch and returns it in full honesty and she knows. She knows she didn't just imagine the brief look in his eyes but it is really there. And she knows that he wants them to touch but is afraid of it at the same time. And she knows that he doesn't want her to know. Cause for all his USAF pilot charm and cocky grin he only has them as a facade; he trained himself to appear like that to everyone else.
She gives always a little more to the touch in moments like that and wishes to never let go again. He lets her get glimpses of himself and she thinks that if she can hold this position long enough she will finally get the whole picture of him.
But the moment gets shattered way too soon as their responsibilities catch up to them. He withdraws from her and gives her a cocky grin while shrugging and turning to leave.
And it is in those moments when her heart breaks for him a little more.