The Argents’ necklace is a key
Since Kate came back, the necklace could make a return too. I’ve always thought there has too be a deeper meaning to the necklace – it is a heirloom after all. I assume the weird shape of the necklace is because it’s made from a melted silver bullet (possibly a bullet belonging to the Argent who killed the Beast of Gevaudan), but it also makes it unique enough to be a key.
“Is there something on the back of it? There’s got to be something, an inscription, an opening anything…”
“No, the thing’s flat, and no, it doesn’t open. There’s nothing in it, on it, around it – nothing.” 1×08, Wolf’s Bane.
When Scott and Derek find the necklace they focus on the actual necklace opening. This could have been included as a clue for when the necklace is brought up again.
Chris and Kate might not even know about the necklace being a key – or maybe Kate has found out recently and wants to find it – she isn’t wearing it when she shoots Derek.
The lock to the key might be in the room Derek, Peter and Braeden visited in 3×14. There were three small boxes on the table, the one on the far right drew Derek’s attention for a moment. There was also a chest to the side of the room – it’s possible that one of these boxes can only be opened by the necklace.
Is this why Derek went back to South America?
What’s in the box?!