Castiel remained silent, his gaze just fixed on Ruby until Sam arrived, and reacted as the angel suspected he would.
"Hello Sam," he spoke quietly before looking at his demon girlfriend, his expression unreadable.
"I followed you yesterday," in answer to her question. "That hex bag you're carrying, it isn't finished. You got sloppy. I saw, Ruby, I saw everything."
He sighed and looked at the ground, struggling to find the words to explain to the Winchesters what was going on.
Ultimately, the brutal truth was the only way.
"Sam's been drinking Ruby's blood. It's been boosting the original power that Azazel gave him as an infant. That's what they've both been hiding. I don't know how long it's been going on for, as I only found out last night, but I can guess. I'm sorry."