He hasn't mentioned that he stopped off to put an angry spirit out of its misery on the way back from his two-day mini-roadtrip. He doesn't mention it because he doesn't want Cas to ask him why they didn't go together. Dean knows the answer will upset the other man, so instead he's bringing back supplies for what's going to be more freedom for Cas and some time they can use to look over the books he's picked up. He hasn't told Cas just how many libraries he's scoured since the wings appeared, but that's not important anyway. What's important is that he needs the answer.
When he steps foot through the door it's as he's slipping his cell back into his pocket. As much as he's not going to say it where anyone else can see or hear it, he's missed Cas too. But that much is obvious in the smile he's wearing as he sets his sights on the other man.
"Did I ever play you that song by Aerosmith?" He knows Cas didn't understand his reference from before, but he can probably change that by digging out the cassette he buried somewhere at the back of his collection.