Re: The missing element in ALL of these is...
They do. They just don't see the point. If the adversary knows a system has a duress code, they can find ways to test it out such as by sending someone else to try it out (possibly a second hostage). That's why the TrueCrypt/VeraCrypt hidden partition concept isn't effective against a determined adversary: they'll simply demand you enter the OTHER password. Seen this way, a duress code may actually make the situation MORE dangerous for someone who doesn't use a duress code because the adversary will assume you do and that you're lying, raising the threat level.