Not disagreeing necessarily, but nukes are harder to move than most think. Uranium & Plutonium are both gamma-emitters, they are present in the triggers of ALL nuclear weapons. And we have something like 9 spy satellites looking for nothing but gamma-emitters, bright flashes, and rocket launch signatures where they don't belong. If the US detected gamma on a ship or aircraft inbound for CONUS, they would be getting a visit from a bunch of guys dressed in black, or a SAM in the arse.
Not that a few megatons burst in the upper atmosphere wouldn't blind those same satellites.....
I do not know for sure, but I greatly doubt that a satellite would detect the radiation emission from a single nuke being moved. I suspect you can also build a container that would deal with ground detection.