They could have been tipped off...
"The reports indicate that they clearly were worried about stow-aways, but then detected the radioactivity. What types of cargo might leave enough radioactive contamination to give off readings that we could detect? There has been talk of having custom inspectors wear devices that can detect nuclear particles, but are these actually in service?"
Good questions that deserve answers...reckon some sort of detection device must be in service for this to be reported.
"How long have we been watching this ship? I haven't heard a single report of this other than on the web."
I heard it on one of the Networks--probably NBC News--last night, but I wasn't in the room and only heard a part of the report. We can probably be pretty safe inspecting only 2% of the vessels if we are willing to profile--racially or otherwise--those ships coming from ports most likely to be accessed by terrorists.
FReegards...MUD
Besides the more suspicious materials, these can also produce significant readings; Phosphate ore, Phosphate based fertilizers, crushed granite stone