Yup. Keep everyone focused on the missile thing and slip a dirty nuke on a shipping container and who will know where it came from.
Any nuke detonated at ground level is a dirty nuke.
They will know via the isotopes from the radiation.
https://www.newscientist.com/article/dn19699-nuclear-bomb-debris-holds-clues-to-who-planted-it/
A nuke in a shipping container would need to be heavily shielded or it would be detected.
Trouble is it is easy to scan to see if any containers on a ship have shielding since a low-level neutron source could not pass through the container.
A nuke in a tank of water would be shielded by the hydrogen atoms.
On a Mars mission the crew would take refuge in a small shielded space inside their water supply in case of a solar flare.
A possible horror story would be to place a nuke in the center of a cement mixer tank and fill it with water...then just drive to the target :-(
Huh. I wonder if the Norks have any particular hatred of Google?
Actually atomic bombs do have a return address on them. The radioactive elements in the bomb are specific to reactors that produced the fissil fuel. We would know who made the bomb rather quickly as in days. However, that is somewhat academic as North Korea would have been reduced to a smoldering ruin even before we had the results.