The existence of a key only being somewhere other than your current location during a border search enables you to legally say "I cannot unlock this device" and move on with your day.

Well even today this guy could have just said 'I will not unlock this device' and they can seize it for some time, but they can't deny you entry. Which is pretty much the same scenario you created except the guy literally can't unlock it (but that doesn't actually matter here, he never _had_ to unlock it).

Could he? To answer properly without possibly committing further possible crimes, he needed a lawyer familiar with whether that's allowable, something he was explicitly denied after asking multiple times. See https://www.courtlistener.com/docket/71998357/21/united-stat...