In the case the evidence was of a crime about to be committed.
And what crime was that, exactly? It isn't a crime, in and of itself, to be radioactive. So what was their reasonable articulable suspicion that a crime (what crime again?) was about to be committed?
Given how rare it is for people's cars to give off radiation, and the potential for such radiation to indicate that a catastrophic crime is about to be committed, a police officer would be highly negligent if he didn't stop and investigate such a car.
The mere presence of radioactivity, in and of itself, is not enough for reasonable articulable suspicion that the person will set a bomb off. It is legal to be radioactive, remember. Driving while radioactive is legal, too. So what else was there that could trigger reasonable suspicion or probable cause?
In this case, I'm guessing nothing at all; just a reliance on peoples' willingness to be helpful to the police and not that they had any legal authority in this instance. I am willing to stand corrected on this, however.
As an example, in states where open carry of a pistol is legal, that, in and of itself, is not enough to rise to the level of reasonable articulable suspicion (and far from probable cause). Will that stop police from harassing you? Some cops, sure. Most will not be deterred from trying to bully your cooperation.