Re: Visible detectors are easy to circumvent . . .
What is required is "intrusive monitoring, without the suspect’s knowledge," otherwise it simply cannot work. And since intrusive monitoring, is federally “illegal” without the suspect’s knowledge (i.e., the suspected perp’s permission), as well as a rend in the US Constitution, it can’t happen (legally).
No metal detector ever stopped the kid laying out in the woods by the school, or across the street on a rooftop with a rifle, and no metal detector ever caught a killer-to-be who happened to carry a plastic knife from freely prowling the hallowed halls of academia.
Dan