I think tracking nuclear material is possible and we have technologies that make it very difficult to move those materials.
What concerns me is that "device" which the North Koreans exploded last week which left a 2.5 mile mushroom cloud and no nuclear signiture!
I read the tin foil sites because in the pre-911 Rumsfeld's Rules, there was one that said "Hire Paranoids - They have a high rate of failure, but the uncover almost all conspiracies" . They are calling that device a Plasma bomb - a new level of destruction and portablilty. Perhaps it was something akin to the Red Mercury device that the Soviets produced.
Unless it was sabotage or an accident, my guess is it was a large conventional explosion to test and calibrate their monitoring equipment.
The US did that prior to the Trinity test.