Indeed, the laws of physics are not secret. Also, the US is lousy with rad detectors. They hang on the walls of train and bus stations in an around big cities. They are present in every airport, major harbor, and border crossing. Many state and local police forces carry detectors in their vehicles. The latter have been responsible for a number of apocryphal stories involving innocent folks detained because they were slightly radioactive subsequent to having nuclear medical procedures (e.g. certain cancer treatments and radiological procedures).