Dean doesn't know exactly where Castiel is sleeping, but it's 'somewhere in the Men of Letters vault', and not 'somewhere in the continental US' so that's an improvement.
He knows that food goes missing from the fridge, that sometimes the shower goes when Sam is asleep, and that books wander around the library from one night to the next. Castiel is definitely alive and living somewhere alongside them.
Dean just hasn't seen him.
Since the day he arrived back to find Castiel sleeping on his bed, since he poured out all the bitter helplessness of the past two years, Dean hasn't seen anything of Castiel. Neither had Sam, but then, Sam barely sees anything that isn't the insides of his own eyelids. He sleep about twenty-three hours a day, and when he's up he huddles by the laptop like the screen is keeping him warm, as if he can actually be of use in the state he's in.
Dean has no idea how to help him. Soup was kind of his only move in the game, and he knows Sam isn't sick, not really. There's nothing in the world that can make him better, no way out but through – but still, he wants to help. That want doesn't go away just because there's nothing for him to do.
While Sam is sleeping, Dean looks through all the rooms in the place, one after the other. He finds boxes of old research papers, icons, dried plants in jars, a pretty large collection of hats, a storage room filled with broken calculators and adding machines, and even a room which has nothing in it, aside from a washer, with a mangle attached – which must be from the 50s at least.
Finally, Dean opens the door of a storage closet and finds himself looking at Castiel.
"Hello Dean," Castiel says, like Dean hasn't found him hiding in a closet.
It's a spacious closet, big enough to contain an army style canvas bunk, a small table and an angel. There are books on the table, beside a glass of water, and hanging from a hook overhead is a hurricane lamp. Castiel is wearing an outfit that he presumably discovered in one of the many boxes all over the place - a pair of black pants and a very severe gray shirt.
"What the hell are you doing in here?"
"Working," Castiel says, then, abruptly, he yawns, surprising himself. Dean notices his red eyes, the dark circles under them.
"What exactly are you working on?"
Castiel shrugs, closes the book he's been holding and returns it to the stack on the table. "These trials that Sam has dedicated himself to, I thought I might find some account of their having been attempted before. There are a number of texts in dead languages in the library, I've been reading them and trying to find information that might otherwise allude you."
"In a closet?"
Castiel shrugs. "I liked the small space."
"Do you also like not seeing us?"
Castiel doesn't quite look at him. His gaze slides just under Dean's left ear. "I thought the two of you should...have some time."
"Like we haven't had years of time to talk and fight and hash out our crappy childhood?"
"I meant time to prepare."
Dean pauses, then steps into the closet and shuts the door behind him. It's dark inside, smells like leather and dust, as if someone had kept boots and coats packed tight in there. Castiel slides over on the little canvas bed, and Dean sits on it dubiously.
"I'm not ever going to be prepared for this...I don't even know that this is," he looks at the books, "what's happening to Sam, you and I both know that no one has done it before. It came off a freaking tablet that no one else has been able to read. Wouldn't be so bad if I could say, the worst thing would be Sam dying...but what happened to Bobby...there are no guarantees about where he's headed."
"I wish I could assure you that heaven was any better than hell," Castiel says, "but, I haven't been there in...well, a long time has passed there."
"I guess I never thought about time passing differently up there," Dean frowns, "when you were...with the war, up there...how long was it?"
Castiel glances to one side, evasively. "A while."
"Time passes in heaven, as it does in Hell."
"That's not an answer."
But Dean could do that math. Six months on earth was forty years in Hell. Cas had been up and down to heaven for a year. That meant eighty years of war.
"So, you only saw us, what? Once every...ten years?" Dean was looking at him fixedly. "How did I not think of that? How did you not tell us?"
"I didn't tell you a great many things," Castiel says quietly, "because they weren't important."
"Well, we're on the same timeline now, and you're still avoiding me." Dean points out. "It's like living with a really big mouse."
Castiel shoots him a concerned look.
"Because you come out at night for food, and I never see you," Dean explains.
"Not because you're making big holes in the walls and chewing up paper...though, if you are, now would be the time to tell me."
Castiel laughs, and it's so unexpected that Dean almost jumps. He watches Castiel's face as the laugh fades to a smile, as the smile changes to a look of benign confusion.
"You're really human right now, aren't you?"
"Almost," Castiel admits. "I'm not devoid of grace but...what I do have is buried deep down, too deep to really be of use."
"Well, while you're vacationing amongst the mud monkeys, you could at least venture out and talk to us once in a while," Dean says, "I mean, Sam is messed up and sleeping all the time, and I can't really do anything to help. I could use someone around to talk to...besides, we gotta work on keeping Naomi away, that bitch gives me the creeps."
"You don't trust her, even after-"
"After she tried to play me?" Dean says. "I don't trust angels, period."
Castiel looked away.
"Hey, you're not just any old angel," Dean says, tapping Castiel on the elbow "you're family. Besides, let's face it, Naomi might have let Bobby in the pearly elevator, but I don't see her getting exploded for me anytime soon."
Castiel looks at him in that way he has that makes Dean wish he could see just what was so interesting in his head.
"Repeatedly exploded," Castiel says at last.
He doesn't point out that Dean said 'me' not 'us'.
"Repeatedly exploded, like a champ."
Castiel laughs again.
"When you get back to being full angel, you should keep doing that, looks good on you."
Castiel blinks, and Dean blinks too, like he hadn't realised he'd been about to say that.
"Come on, family don't sit in closets researching all day," Dean got up and opened the door. "Besides, I made a vat of goddamn soup, and someone's gonna help me eat it. Consider it your Winchester initiation."
They leave the little room and Dean talks about random crap the whole time he's fixing the soup and setting out spoons and bowls. He's talking about when his Dad used to make them soup, the time he dumped a whole jar of pepper into it, and how the addition to the recipe kind of stuck.
He's talking, but he's thinking too. Thinking that, even if he can only stand beside Sam when whatever happens, happens. Even though there's nothing out there he can research to keep his brother safe, he can damn well hit the books looking for a way to keep Cas safe, or as safe as he can be.