Hospitals tend to notice clusters of people dying of radiation poisoning - at least in places with hospitals.

In places without hospitals - I'm sure the contents of a lot of these machines end up being dumped in the 3rd world, to be 'recycled' like most toxic waste

Only if it is acute poisoning and only if they cluster in time and space. Devices as nasty as this, of course, would be detected, but less deadly materials would easily go undetected until someone, 50 years later, notices something like leukemia being 20% more deadly in a given region than the surrounding areas.