@ai6yr
So the nettlesome question is: how does a large amount of zinc get contaminated with Caesium-137 during the recycling process?

It seems likely that they would have been stored, transported, and/or melted down together, which suggests they were also potentially collected together. So perhaps not merely a stray stolen gauge, but a more extensive dismantling of something with substantial amounts of galvanized materials (fences, pipes, steel building beams) in the vicinity.