As an example of how that can end see the Goiânia accident. There the same stuff, caesium-137 inside a secured box, was just thrown away in a trash dump. Then someone found the box, wondered what valuables are inside such a closely locked thing, broke it open and... played around with and spread the glowing magic powder to the neighbourhood. A lot of people got severe radiation doses, it needed very serious clean-up, and people died.

> And how is it lost off a truck if it’s so dangerous?

The honest answer is: because people are sometimes idiots and don't follow adequate safety measures. Somebody screwed up. In the above incident, that wouldn't be the guy who found it (you can't expect random people to know about caesum-137 and its use), but whoever carelessly threw it away.