The walls are still naked, the bed has no covers and nothing but the occasional borrowed book from the Bunker’s library resting on the nightstand indicates that anyone even lives in this room.
But there’s a hidden box under the bed, a box Dean only found by accident, when he was returning Cas’ freshly washed trenchcoat. The box is empty - except for two photos (one of Claire, one of Sam and Dean) and a small, frayed black feather.
The box stays in the room, under the bed, even when Cas isn’t there. It makes Dean breathe lighter, to know that Cas leaves something here that is valuable to him. That way he knows Cas is coming back sooner or later.
And he does.
Because there’s something valuable waiting for him back home, in the Bunker.