well, yeah, by "suitcase nuke" i don't mean a slim leather briefcase, but a small enough size that it could be easily carried in the trunk of a car. even a little car.
you can mask the bomb from radiation detection, but you can't mask the mask, and that's one of the major things they look for in checked baggage/air freight. "hey, look! a box of the perfect density so we can't see inside or detect radiation!"