Sam knows a code word when he hears one. Hell, he and Dean have about a hundred or so words and phrases and hand gestures that they’ve accumulated over the years to make a language all of their own. It’s not hard to figure out when something means something else entirely.
Which is how Sam finds out about Dean and Cas’ “secret” relationship.
The first time Dean asked Cas if he wanted to “play Scrabble,” Sam didn’t really think much of it. Dean and Cas disappeared to do random shit all the time. They were, after all, best friends and pretty much attached at the hip.
But then Dean asked again a couple days later. Then Cas asked. Then Sam realized.
They would disappear into Dean’s room to “play Scrabble,” and then they would come out two hours later looking angry or exhausted or smug or any other emotion to describe that they had just fucked.
Sam elected to ignore it. He didn’t really care if Dean and Cas were secretly fucking as long as they were happy. If things got serious, they would tell him.
That is, until one day Sam walks into Dean’s room without waiting for a “come in” and finds Dean and Cas–
“What the fuck are you guys doing?”
They both look up at him with wide eyes. Dean holds a little wooden letter in his hand and responds, “What does it look like we’re doing?”
“It looks like you’re playing Scrabble.”
“Yeah. Didn’t you hear us say we were going to play Scrabble like an hour ago?” Dean puts his letter down, and Sam sees the word “bullet” written on the board.
“So…that wasn’t…a code word?”
Cas answers this time, “Why would Dean and I need a code word at home when it is only the three of us here?”
“Because you’re–you’re–never mind.” Sam slams the door behind him before they can say anything else.
Muffled through the door, he hears Dean say, “Oh my god, dude, I think Sammy thought you and I were having sex.”