Though there's always the possibility of a new scenario that only requires the base game, any cards left unused in the base four scenarios could be included for use in scenarios requiring first edition boxes, Suppressed Memories, or Recurring Nightmares. Those boxes don't include new cards.
I can confirm the obscure appearance of the handcuffs but I think primarily they will be used as part of a mechanic in one of the two upcoming scenarios. At a guess, the Call of the Wild one, because the 4 NPCs from that set have not yet been used either.
It seems very obvious to me that these scenarios were designed alongside the base game ones and held back. There are also a number of 2nd edition tiles not yet used that will presumably be used in these scenarios. Ghosts to date are barely used and never 2 at once. So the base game content was clearly designed to incorporate these scenarios.