Without knowing exactly what they did, it's reasonable to assume they searched his car. Generally, this requires a warrant unless it's incident to an arrest, and even then, there are limits.
There's not much legal precentdent either way as to whether or not slight radioactivity consitutes probable cause, but it's a very worrying slippery slope if it does. Cop wants to harras you? All he has to do is put a few drops of some nuclear medicine on your bumper (or worse, on your person) and you'll be stopped and searched thoroughly, just because he thinks you're guilty.
Hell, he can just claim you registered, search your car illegally and haul you in for whatever he finds.
TL;DR: It's a slippery slope for due process.