The problem is differentiation from background. A release from NK or Russia, by the time it hits your Geiger counter, will likely be so low that it will be los

The problem is differentiation from background. A release from NK or Russia, by the time it hits your Geiger counter, will likely be so low that it will be lost in the noise of background radiation. And local sources (such as radon gas) would mask anything but significant nearby events. The reason why France identified the release is because their detectors can determine different radioisotopes. It's a little like the difference between a photovoltaic cell and a camera. Your Geiger counter can tell when the "light's on" (presence of radiation) but it takes a different instrument to determine that it's Ruthenium 106.