Significance of the key on his neck?
So throughout the main story Jack is wearing a rather mundane little key on a thong around his neck. Did anyone catch what the significance of this might be? I didn't notice him or another character refer to it. I don't recall anything about it from his childhood or teen flashbacks, nor does it remain in his prologue/epilogue monster-slayer getup. Neither did I see any reference in the deleted scenes.
Seems like an odd detail for the director (or costumer) to include for no reason. Or did I just miss the reference somewhere? Maybe it's some kind of inside joke?