If there were suitcase nukes that went missing, they would well be past their shelf life by now. I was a nuclear weapons officer in the military, and know a little about the small critters that go boom. The degree of nuclear fuel refinement that is necessary gives them a shelf life that is in the six year range. Beyond that and their yield deminishes dramatically and within three years they become useless. Even with modern innovations that have come down the pike since I last served as an Officer (1983), the shelf life parameters can not change that much as refinement of fuel is refinement of fuel and that has not changed..
Okay so they'll set off a nuclear dud.
Perhaps it won't kill everyone in Manhattan but you could still take your eye out with that thing!