Cs-137 is dangerous to animals (including humans) in close proximity, precisely because of the gamma radiation it emits. But, like any radiative emission, the p

Cs-137 is dangerous to animals (including humans) in close proximity, precisely because of the gamma radiation it emits. But, like any radiative emission, the power density drops off with the inverse square of distance from the source, which makes it detectable well beyond any range at which it poses a hazard. Think of stadium lights: if you stand five feet in front of one, it will blind you, while from a mile away, it's harmless but you still can see it. With gamma radiation you need a detector (ie a Geiger counter), and with an at the moment presumably unshielded Cs-137 source, a reasonably sensitive detector will indeed pick it up at some distance. So, at worst, if you put enough people in trucks with Geiger counters along the same route the original truck followed, sooner or later you will find the lost source. That said, this method while effective may take some time, and the major concern here may not be so much the direct radiation emission from the intact source, as the possibility of damage leading to dispersal of its contents. In the Goiânia incident of 1987 [1], scrappers removed a Cs-137 source from an improperly decommissioned radiotherapy machine and broke it open, dispersing the loose isotope material it contained in ignorance of the danger it posed. Several people died, others were badly injured, and at least one house and a lot of soil had to be removed for disposal. If this lost Australian source is similar in design to that one, warning people about it while looking for it is an eminently sensible public safety measure. [1] https://en.m.wikipedia.org/wiki/Goiânia_accident https://en.m.wikipedia.org/wiki/Goiânia_accident