Adding a little clarification to rutless's post:
In the Cold War era, there were nuclear artillery shells, land mines, recoiless rifle rounds, pathfinder rockets, torpedoes, and lord knows what else.
When the .gov discussing securing nuclear weapons, most of the time they are talking about ballistic and guided missiles. The US spends a lot of money to make sure these things are being guarded, disassembled, or scrapped and some type of records are being kept.
As for the nuclear artillery shells, land mines, torpedos, etc, its anyones guess. I dont think they are as heavily guarded and accounted for as the missiles are, and I dont think Paul Williams has accounted for that in his book. It would be a lot easier for one of these to disappear from an unguarded warehouse at an abandoned military facility in the former USSR than any other type of nuclear weaponry.
A lot of people have "suitcase nukes" as their only frame of reference when discussing portable nuclear weapons, and they dont realize there are many types of miniaturized nuclear weapons that were developed during the Cold War.
About ten years ago, there was a lot of talk about unsecured nuclear weapons falling into the hands of the Russian Mafia, and I suspect something got out. IIRC, some Russian Mafia types were busted with nuclear materials about 1995 or so.
Thats my .02...