You were the child of Dean and an angel. Though your mother had raised you as a human, you still had some nephilim powers, like being able to see angels true faces. Though he'd put a spell on your mother for her to still see Castiel, he couldn't do much about you.

So, while Sam, Dean and your mother were out hunting, they tasked Lucifer, who they believed Castiel, with watching you. And the only way you were looking at him didn't go unnoticed.

"What are you looking at?" Lucifer asked.