Yeah, and their nuke designer walked into an embassy and didn't come out!

Seriously, there are people who make it their business to know who can make a nuke and where they are on the production schedule. Unfortunately, once you've got a nuclear factory going, you are in the mass production game. The purification process whether it is from centrifuges or breeder reactors is a continuous one. Again, this is why it was a good idea to make an agreement with a regime actively seeking to go that route to severely discourage them while there was time.

On the other hand, there are such things as suitcase nukes or the following which I would not read on a full stomach. Someone might be able to steel or buy one, but again, there should be people around keeping track.