The first thing Sam looked for when they returned to the apartment was the phone. He assumed that this was where Dean had been when he'd called, because he'd found the cuffs, and the cuffs were here-- sure enough, there they were, on the floor near the phone; the two things Dean had been holding when he'd been taken. He didn't see any signs of a struggle, but he did a search of the room, just in case: for sulfur, mainly, because the last time he'd disappeared that fast was when yellow eyes had come for him and all the other psychic kids all those years ago. There was always the possibility that a demon could've snatched Dean and taken him to a place with angel warding, which would make it impossible for Cas to find-- but even if that were true, he'd have expected Cas to still be aware that Dean existed somewhere in this universe. "We're sure that's the only explanation, right?" he said, just to be sure. "No chance anyone's managing to hide him from you?"
It wasn't that he didn't trust Cas-- he did trust him, quite a bit in fact-- but at the moment he would have preferred to believe that his brother was kidnapped by demons and the angel had made a mistake. It was better than the alternative of him simply being gone. Judging from the dejected look on Cas's face, he probably felt the same way-- and one glimpse of it answered Sam's question, really.
"Damn it," he repeated, more quietly. He sat down on the couch and raised a hand to rub at his mouth, before resting his elbows on his knees. He stared at the phone on the floor for a moment before shaking his head and raising the same hand to rub his forehead. Dean was gone, and he needed to let that sink in before he could figure out how to deal with it. The panic had subsided with the finality of it, to be left by... what? He didn't know. Numb shock, for the moment.
Then Cas spoke, and he gave a mirthless chuckle. "Yeah. Maybe. Or maybe he's done here. Or it's coming for all of us, maybe our time here is up." If that was the case, it was cruel to only take one at a time. He glanced up. "From now on, let's check in every few hours, okay? Every two hours. If you don't show up, or if you discover I'm not here, then I'll know." He swallowed. "If I'm sleeping, you can wake me up. Or do that thing where you show up in my dreams."
At the moment, he really wasn't concerned with whether that would mean Cas interrupted anything. Because his dreams weren't going to be anything fun tonight, not with Dean gone, and he would warn Veronica-- he figured she'd understand that he needed to know quickly if his friend disappeared. Cas being an angel that could just disappear on his own for lengths of time would make it difficult to figure out if he was still here or not.