The reference term for lost US warheads are broken arrows. ;) https://en.m.wikipedia.org/wiki/List_of_military_nuclear_accidents

What's interesting is that there were several us and Russian vessels sunk with nukes aboard. No recovery took place because they were deemed not recoverable at the time. Modern deep sea salvage tech has forced many countries to reevaluate these losses. Some of them found the nukes were gone during reevaluation. I remember reading about it. Between that stuff and the nukes lost during the Soviet Union collapse, there are lots of avenues for a loose nuke to end up in evil hands. I think Uranium 1 material is going to be the play because it exposes the traitors within.