We keep our Cesium inventory in a locked safe in a locked room in a locked department in a locked hospital. We don't have 24 hour armed sentries stationed at the door, but it is highly unlikely,
but possible that sources could be stolen. It would be tedious to break the stainless steel tubes open to get the cesium, but theoretically it could be done and mixed with explosives to disperse it much like anthrax spores. It would take a lot to achieve a dose level that would be
immediately harmful, but would cost a tremendous amount to clean up. The main danger would be inhalation of the dust, or remaining on site for a long enough time to get a significant gamma dose. The long term environmental risk is getting into the food chain, genetic damage, and long term carcinogenesis.
By the way, potassium iodide would not provide any significant radiation protection from from this type of radiation exposure. I have been reading other threads that are filled with misinformation about what KI (potassium iodide) can and cannot do for radioprotection. Its only use is with fissionable byproducts of a full nuclear chain reaction which produces Iodine-131 which is taken up by the thyroid and can increase the risk of thyroid tumors (benign and malignant) after a latent period of up to 15 years (not the type of problem with a "dirty" nuke). If there is a fissionable nuclear attack, concern about cancers 10-15 years in the future will probably be the least of anyone's concern. KI will not protect the bone marrow from gamma radiation, the lungs from inhaled plutonium dust, or the skin from radiation burns secondary to contact from fallout.
I wouldn't worry too much about plutonium dust either from a nuclear blast or from a radiological bomb. Plutonium of just the right particle size to be inhaled is difficult to make, and without special blowers Batelle Northwest laboratory researchers found they couldn't cause cancer in rats because the plutonium plated out on the cage surfaces because of its high density. Plutonium also doesn't react chemically with too many substances, so it is not likely to find its way through the food chain. Injested, Plutonium is about as deadly as caffeine, since it doesn't pass across the gastrointestinal tract very well.
Just as in Nagasaki and Hiroshima, the number of deaths from blast and fire from a small nuclear weapon device will vastly outweigh the excess cancer deaths from radiation poisoning over the next 30 years.