Dean didn't talk about things when things were wrong -- at least, not when things were actually serious. He didn't express his feelings if he felt like other people would get hurt, he didn't talk about things that really ate at him, and there was something wrong now. It was small and subtle, something strange in his behavior, in his need to behave normally -- it was normal, but just slightly off from normal, too.
Cas had been living with humans for months now, and while he didn't really understand all of the subtleties, he knew Dean. He knew when Dean was covering something up. He saw it when he was kicked out of the bunker, saw it when Dean left him again at the gas station. He knew now what all of that was, that it had been about Sam and the angel inside of him, but this?
This was something else.
Cas narrowed his eyes, watching Sam closely. "You've noticed, haven't you."