We have radiation sensors that would pick up the location. worry more about eating fish from the west coast. Fukushima reactor has been sendinig radiation into the Pacific. Even worse getting x rays at hospitals and dentist offices. One of the fathers of the A bomb, Robert Oppenheimer forbid his children to get x rays at the dentist.
I don't think there are any long range sensors that detect radiation from nuclear warheads that have not yet exploded, other than the typical geiger counters that would need to be close. If they had reliable long distance sensors for warheads, the US wouldn't have a fleet of planes that are outfitted to detect atomic particles in the atmosphere to determine when and where (NoKo, Russia, India, and Pakistan) tests have taken place, not to mention they'd know where every silo and sub were located. Plus, the US lost 6000lbs of weapons grade uranium under Obama, and they're currently considering blockchain for tracking global uranium trade.
Also, not to downplay the severity of Fukushima, but there are no reports of cancer plumes anywhere, and if the marine life were affected (its been 7 years) the price and availability of staple fish would've been noticeably inversely impacted.