It's the wind chimes that make Dean realize.

Not right away, and not just one or two, either. It takes a while before Dean notices the details and figures out what they add up to. When he starts watching Cas more closely - and not just warily, but with actual interest - Dean begins to notice that there's a constant breeze blowing around him. It's not forceful and no one just casually glancing Cas's way would have caught it. But, yeah, Dean does, and it's the wind chimes that bring it home.

They're standing outside some random ass gas station in some random ass little town and Cas is standing maybe a foot away from the frame of the porch that holds a small group of wind chimes. Dean's come to terms with the fact that Cas doesn't know a lot - if any - normal human manners, so he can safely stare at the angel with no repercussions. So, when Dean hears the high pitch tinkling sound that only wind chimes can give off, he turns and stares directly at Cas. The angel doesn't seem to notice the attention, as he's staring at something inside the store window with a scrutinizing look on his face.

The breeze that's moving the wind chimes is coming constantly, in the same pattern. The chimes gently swing forward, backward, and repeat. There's no conflicting breezes to screw with the rhythm and the force of it never changes. Dean watches Cas, and watches the chimes, and it hits him. Wings. Cas's wings are causing the breezes. Dean doesn't know why it didn't occur to him earlier, maybe because he'd always thought that if Cas's wings were hanging around on this plain of existence, they'd be folded against his back. Maybe he doesn't notice when people walk through them? Maybe he likes freeing his wings up a bit and not having them packed against his back.

Dean absently wonders what it feels like - to be in a vessel, but still have wings as if you were in your true form. It probably doesn't feel the same, Dean knows, but then he has to wonder what it's like to feel yourself change when you enter something like a vessel. Dean's glad he's never gonna find out what that's like.

So, now Dean stares at the space where he thinks Cas's wings would be if they were visible. He watches what the breezes do and forms an image in his mind, an outline of wings creating those breezes and riding the normal, non-angelic winds that come and go on occasion. He sees them as shadows, maybe more feathery, because of the first time he met Cas, and he saw the outline of his wings. They were wonderful, and he wishes he could see them again. He wishes he could see them spread wide or folded back or even just swaying in the wind like that always seem to be.

And if Dean smiles every time he hears wind chimes, looks over at Cas if he's there and just stares at the chimes themselves if he isn't, Dean figures that's not an entirely chick-flick thing to do. Even though it really, really is.