He picked it up. Of course he did. Why not? He couldn't just leave it there, in some motel, to rot in a trashcan. No matter what Cas, or Dean said, it was important.
He keeps it hidden in his pocket, taking it out only when he knows he's got all the time in the world. The time to memorize every shape and hole and crack. To tighten his hold around it and feel the familiar weight, feel it press against his hand.
When times get really bad again, he stares at it for hours. The glinting of the golden, rusted color shines in the darkness as he remembers the tine he first saw it. And when he gave it to Dean...
Sometimes he thinks he should return it to Dean. It belongs with Dean, not Sam. Dean should be the one holding it, the amulet resting against his chest.
But he never does give it back.
Maybe because he is scared that Dean will just throw it away, saying it's useless. What if he gets angry? What if he tells him he shouldn't have picked it?
That night with the musical, when Dean got another version of the samulet, those thoughts became void. And he had been so happy. To see Dean hold it, look at it. But he still did not give it back.
What would Dean do if he ever saw it again? Would he put it on? Would he stare at Sam like he was mental for picking it up from that trash all those years ago? Maybe he would be happy.
Sam didn't know. But he would not risk it. He could not loose it again. But he still keeps it with him. To the grave.