Lucifer was in the cage, when he convinced Castiel to let him possess him. No one knew, not Sam, not Dean, and not even you, Castiel’s partner. You and Castiel had been dating for a few months now, which made it all the more surprising he didn’t at least talk to you about it first. But that’s also why you didn’t suspect him.

You all got back to the Bunker, no one suspecting that Castiel was actually Lucifer right now. You all went to separate places, you and ‘Castiel’ going to your room.