Reminds me of when they started doing bag checks to ride te NYC subway. The official position was if you volunteer to ride you give implied consent to be searched. Nobody has to ride the subway...
Then, while listening to the Lionel show on local talk radio in NYC a NYPD office called in and was asked about what was justifiable cause for a search. His answer was "anyone who turns around and not ride rather than be searched.". So you decide not to undergo a voluntary search and that act justifies an involuntary search... That is exactly how this technology would be implemented on a day to day basis.
This technology may not work well now but I bet enoug money is being injected to develop it that this will be common within 20 years. Legislators and Presidents will side with it as "Public Safety" no matter what party they come from. No elected official wants to be known as having voted against a measure which would have spotted a rampage killer. If the SCOTUS doesn't clearly smack this down as an illegal search be prepared for it to become a part of life in a generation.
"Religions are all alike - founded upon fables and mythologies." Thomas Jefferson
"The way to see by faith is to shut the eye of reason." Benjamin Franklin