"There would be no hard evidence linking the source of the attack." If are willing to take seriously a movie, Sum of all Fears, then for each ball of plutonium

"There would be no hard evidence linking the source of the attack." If are willing to take seriously a movie, Sum of all Fears, then for each ball of plutonium, just take the distribution of its isotopes. If have a catalog of all such balls of plutonium, then, for any one ball of plutonium, its distribution, looked up in the catalog, will identify where and when that plutonium was made. Similarly for any bomb from such a ball of plutonium -- just get the distribution of the isotopes in the fallout from the explosion, look up in the catalog, and know the source of the plutonium. Maybe from the nuclear weapons treaties, the catalog was developed. If all that is approximately the case, then whenever a plutonium bomb goes off, will know the source. Next, it may be the case that, if a missile is launched at sea, then US satellites will detect the launch and start a search -- e.g., by air, dropping sonar buoys with active sonar -- to find the submarine. It may be the case that the submarine trying to get away is limited to a speed of only about 6 knots since going faster might generate enough flow noise for the submarine to be heard and located nearly anywhere in the ocean. If the submarine is limited to such a slow speed, then it would appear that the airplane and sonar buoys would be able to find the submarine easily enough. Next, it may be the case that the US knows the location, 24 x 7, of every submarine in the world able to launch a nuclear missile. Net, for a missile with a plutonium warhead to be launched at sea and the submarine to get away without being detected might be nearly impossible.