About what I thought. I recall reading of systems that could detect such radiation but that they were not widely deployed, not terribly portable and were a budget line concern. Detecting neutrons, neutrinos and the products of their decay is not for something that you plug into your smartphone for a walk amongst the containers. It may be possible to fit such a detector on the back of a truck and drive through the isles of stack containers in a port or sweep across the stacks of containers on a ship entering port. Knowledge that such a system existed in a given port would raise doubts by a potential state sponsor, even if the chance of it being used was low.

"It is not necessary to have hope in order to persevere."