It was likely just another of Ruby's lies. Clearly it suited her, undoubted, agenda to have Castiel doubt Dean. He did not. His belief in his charge was steadfast. He believed Dean would choose right in the end. If anyone challenged his belief as naive he would dismiss there opinion as ignorant of Dean's nature. Castiel knew a righteous man when he saw one and Dean Winchester shone brighter than most. Something the human himself would, no doubt, deny flatly.
But losing integrity and losing hope were two different things. Was it possible? It was true Dean would much rather not have anything to do with the angels, the mission, the apocalypse. He had been very vocal on that.
This was a real concern. He needed to find Dean. He needed to remind him of the cost of giving up, needed to reassure him he was already enough, he already had everything he needed.
He was silent for some time before he answered Ruby. Why was she asking his permission to move away? Was she still afraid she was going to kill her? He was more irritated than concerned.
"Go," he said. "You will not doubt tell Sam I am here. He will find out soon enough anyway. We will also meet again," Castiel said matter-of-factly. Castiel was not even sure why he didn't kill Ruby here and now. It was likely everything she had sold Sam Winchester was a lie however Castiel could not choose for him and killing Ruby would mean doing just that. It could wait until she gave him a good reason.
That was unlikely to take long.
He vanished with the sound of wings beating the breeze.