1. Santa Fe, NM
The three of them split up at the cemetery; there’s more than one Jose Martinez buried there, and they have to check the dates to make sure they salt and burn the dude who's been haunting the mall parking lot across the street. It's Sam who finds the grave, drops a small reflective beacon so they can find it again, and sets off in search of Dean and Cas.
Who have, evidently, never separated. From a distance, Sam thinks at first they're just doing their lack-of-personal-space thing, their shadowy figures overlapping in the dark. But then the moon comes out from behind a cloud, and he clearly sees Dean take Cas's face in his hands and kiss him on the mouth.
Sam stops in his tracks, because huh, that's new. Not really a surprise so much as a...okay, this is a weird thought, but it's almost a moment of narrative satisfaction? Like the part of the book where the the mystery gets solved or the hero slays the villain. Startling, but inevitable.
But also none of his business. So he makes all the noise he can manage as he heads towards them, snapping twigs and tripping over a flat gravestone with a too-loud curse. By the time he reaches them, they're standing a respectable foot apart, and though Cas's hair is mussed even more than usual and Dean's breathing a little hard, it's easy for everyone to pretend nothing's happened. So they do.
2. Derby, KS
"So get this, guys," says Sam as he opens the door to the motel room, and narrowly misses getting hit in the face by Dean's shirt.
Cas is sitting on the kitchenette counter with Dean standing between his thighs, his trenchcoat sleeves bunched down around his elbows, his shirt unbuttoned, his tie still absurdly hanging on his bared chest. Dean's shirt is now dangling off Sam's shoulder, and the telltale jingle as he turns his head to face Sam means his belt is unbuckled as well. They're both panting, there's a mark on Cas's collarbone that looks suspiciously like a bite, and though Cas is now looking at Sam as well, he does not remove his hands from Dean's ass.
"Uh, we didn't expect you back so soon," Dean says after a moment of stunned silence.
"Obviously," is all Sam can think to say.
"Sam, it's not, uh—"
"If you say 'it's not what it looks like,' I'm gonna scream," Sam says. "It's exactly what it looks like, you're making out with Cas, and that's fine as far as it goes, I'm happy for you guys. But, uh, try to keep the PDAs to a minimum? As a personal favor?"
Dean mutters an apology, nodding, and Sam tosses back his shirt.
3. Nashville, TN
Sam wakes up at the ass crack of dawn and goes to brush his teeth at the sink outside the bathroom proper. The door's ajar, and he can hear the shower running, so he knows Dean's up. Good, they've got twelve hours to drive today.
He's spitting toothpaste into the sink when he hears a low moan that tells him Dean is enjoying some alone time in the shower; he's reaching for the doorknob to pull the door shut when he hears an entirely different moan and realizes that, no, Dean is, in fact, not alone in there. He's showering with an angel, and there's probably not a lot of actual hygiene going on.
It's not technically a PDA, but he still flushes the toilet before he closes the door on Dean's shouted "Goddammit, Sam!"
4. Ferndale, MI
Dean had no interest in hitting the little used bookstore, so Sam went in alone while his brother stayed in the car. He's carrying some great finds—some Bradbury with cool 70s covers, a 2nd-edition Complete Wizards’ Handbook—as he approaches the Impala; Dean's dozing in the driver's seat, head lolling back on the headrest.
He swings open the passenger door, and holy shit Dean is not asleep. Because Cas is a khaki shape stretched prone across the seat, and his head is definitely in Dean's lap, and Dean's hands are buried in his hair, and there is definitely a blow job in progress and oh my God that is definitely an orgasm noise leaving Dean's mouth right now.
Sam slams the door and covers his eyes, and just stands there, blind and in dire need of brain bleach, until he jumps at a gentle hand on his shoulder.
It's Cas, and he is thank God fully dressed, and Sam is pointedly not thinking about the state of his mouth right now. "I'm sorry, Sam," says Cas quietly. "You were not intended to see that."
"No shit! That is my car too, Cas, I sit on those seats. I'm about to sit in those seats for hours, and now all I'm gonna be able to think about the whole ride is you—you going down on my brother!"
"Sam, we're really sorry," calls Dean through the window. He hasn't moved, and Sam doesn't look at him.
"The Impala is off limits beyond first base from now on, okay? I'm calling it."
"We will honor that," Cas assures him, and Sam finally shoots a glance Dean's way to see him nod, chagrin etched across his face.
"You better," says Sam. "I've got weapons."
5. Stamford, CT
Sam's not sure what wakes him up until he hears it again. It's a whimper from Dean, followed by a low, breathy, "Cas, baby, yes," and he is in the same room, for fuck's sake, this is clearly inappropriate.
He's about to turn over and say so when he hears Dean whimper again; it's different, there's a catch in his throat that's almost a sob, and then Dean says, "I don't want you to go, Cas, please don't leave me."
It's such a strange thing to hear in Dean's voice, so vulnerable, that it stops him. Sam's only heard Dean talk like that on a few occasions, and he knows it's difficult for him to do so. He feels creepy as hell about it, but he can't bring himself to interrupt, just lies there, frozen.
"I never want to leave you, Dean," he hears Cas say.
"But you do," says Dean. "You do, all the time—oh my God, keep doing that," and he abandons words for a moment in favor of muffled gasps.
Then he speaks again, "I need you, Cas, I need you here." He makes a noise of protest; evidently Cas has stopped doing...that.
"Is that what you mean to say, Dean? That you need me?"
"No," says Dean, "no, please, come on, don't make me."
"I love you, Dean," Cas says. "I love you."
There are sounds of kissing, and Sam starts thinking again he really needs to say something, and then he hears Dean say, so softly it's almost not words at all, "Love you, Cas. Love you so much."
Sam puts his pillow over his head and resolves to talk to them in the morning.