"Did you ever see a follow up article on how we detected the radiation?"
My hunch is that such information would have the highest security rating there is. Knowledge of how the detectors work could help circumvent them.
If I was designing the system, I'd build a gamma ray detector. Gamma rays are very very hard to completely block. (I'd try for neutrinos if the tech existed -- maybe the DOD has it. The advantage there: Neutrinos shoot throught the entire planet without stopping. They are IMPOSSIBLE to block in any significant manner. Unfortunately, they are also VERY hard to detect.)
That was merely a rhetorical question. Obviously no one is going to tell anyone how we detected radiation aboard a ship at sea. No doubt we could have done it from space, or maybe below the surface. However we did it the fact that we could do it was revealed at that time.