When he flew somewhere, Castiel stayed invisible for a few moments to get a grasp on his surroundings and sometimes, he accidentally overheard or saw things. He overheard conversations not meant for his ears and saw things people didn't want him to.
Today for example, he flew to the bunker to check on Dean and Sam. He landed in the library and at first nothing seemed to be out of the ordinary. Sam was sitting in his usual spot, piles of books surrounding him. Just as he was going to make his presence known, he heard it. Sam was whispering something in a strange voice. Castiel stepped closer and saw Sam playing with two dolls.
„Oh Dean I love you so much. I wish I could tell you, but a guy like you could never love someone like me. I'll just stare at you from afar" Sam whispered. He then moved the other doll closer to the first and whispered: „Dean! Please tell me you didn't hear that!" „I heard everything and I love you too. Let's kiss!"
Castiel had heard enough. He knew something had been wrong with Sam. Let's if Dean will still laugh at him for worrying about Sam's mental health.
Castiel flew out of the room before he could hear Sam whispering: „I love you Dean" „I love you too Cas."