It's not razing fires and blaring horns that she feels when she sees him back in the bullpen. It's more like razor sharp edges silently piercing her carefully reconstructed fort at the sight of his familiar two piece dark suits. She's smiling at him, despite wanting to break down and run away from his searing smile. It's not that she's scared of him, but after all the time she's spent piecing together her life before he intruded (but somehow was welcomed), there was a certain numbing quality she had sewn into the walls of her mind. A delicious feeling after the two weeks spent cringing away from even the sight of pancakes (It was unasked but she appreciated the gesture till this day.)
She won't admit it but his smile brought back memories flooding, bursting its' fragile sticks and stones beaver dam. The time when he said "Because you are tall." And in that one absolutely ridiculous, insane (sensible, proud) phrase he managed to convey all his confidence that she was going to get through this case and the next and the next. The offering of coffee and bear claw every morning that even without her saying, he would painstakingly hunt the whole city for every single god damn day. Or the way he would poke and prod her tatters of sanity with his egoistical words just to keep her on her toes. When he broke down that door the day her home became a soot coated, scorch marked apartment which would remain just that forevermore, expressing all his fears of loss in that single "KATE!" she knew that just maybe they were more than friends. But he left her in this avenue filled with the spectres of the past and she's all alone to fight it. In the midst of all the never ending battles, she comes to terms that this… whatever she had with him was merely just that; a whatever, an unfulfilled, haunted, broken, unrealised whatever.
It's not like she did not expect him to be so… dense, there was simply no other word for it. She did say to him "Until fall," echoing his sentiments about how 'until tomorrow' seemed more hopeful. But it was never meant to be this way, his apprehending, multiple apprehending, trust him to be the only idiot in history to land in such a fix. She was willing him to not join this case (all the same, she was wishing he would stay, would never leave). But she's no longer Katie of the pink little dresses, childish believes and fairy tale kisses. She's Beckett, the steel eyed, stalwart, persistent, hard edged detective. Though she wishes so hard, that stars in their earnestness to shine pale in comparison, she could be Katherine, the beloved, popular, consummate Broadway actress she could (should) have been. But she will only ever be Kate on those rare occasions and she doesn't want to admit it but she likes being Castle's version of Kate rather than Will's or Tom's. His is the best because in it, she's flawed and stronger because of it, a bleeding, sore human who refuses to show signs of faltering, but as close to perfect as she can get. But he only ever said it a few times, granting her on each of those occasions a glimpse into what could have been.
Despite all of that, when he had stood facing off her and the unsuspected demon behind her back (it's after she turns around to see the suspect on the ground behind her does she realise how close she came to losing Castle) and gunned the perp down, she smiled at the recollection of it. Because he had her back at the end of it all and that's what mattered. And it gives this thing they have going on a chance. For the moment though, she's content with gliding away when he gets too close and sauntering to him if he were to lose the rhythm of this jig they have going on.
It's her way of demonstrating "Until tomorrow."