In that scenario, I would hope the "beyond a reasonable doubt" standard would apply

No need for hope here. "Beyond a reasonable doubt" is a legal standard that applies to evidence presented in criminal prosecutions. It does not apply to investigations or, for example, things like being put on the no-fly list. Or the "next target for the drone assassination" list.

Moreover, at a border crossing the Fourth Amendment basically does not apply, too. A border control official can search all your belongings including your electronic devices without needing to show any cause, never mind about "reasonable doubt". At the border, TSA can trawl through your laptop or phone at will.