Just a theory, but that absolutely would make sense.
They would NEVER tell us.
It would make sense if the raw material were Plutonium; you can use remote sensing with a Uranium-based weapon.
With Plutonium you gotta be SUPER SUPER close.
Hmmm...vexing.
Everywhere you turn in the military you find time and attention (even on deck) spent on completely STUPID stuff, but then again NO ONE believe in the transparency claims of the US Gummint, these days (as in if there were bomb potential).
So both potential explanations make sense.
Hmmm....
A dirty bomb in a container or tanker? Meh, no biggie, not worth upsetting the "little people" by mentioning the possibility or the fact that we consider it a real enough threat to be actively looking for anything like that.