To become a real threat, it would have involved so many scientists and technicians and money that it would have easily been spotted.

You could make a pretty good case that a nuclear program would fall into this category, although it should be remembered that Saddam successfully hid a large nuclear program from the U.N. before the first Gulf War. Note that the most serious threat was deemed to be the acquisition of already-refined uranium to be built into warheads, a process that is much easier to hide than the equipment required to refine uranium into bomb material.

Chemical programs would be less easy to spot than nuclear ones, and biological weapons programs could be done with very few people and facilities.

It's already been established that Saddam was working with a highly decentralized organization system for WMD, potential or otherwise.