My guess is it'll eventually be traced back to improperly disposed of Cs-137 source. This wouldn't be the first time [1] [2].

There was also a famous case in the 80s where a scrapyard in Mexico sent some steel contaminated with Cobalt-60 to a foundry where it was melted down into rebar. It was detected when a truck transporting rebar to a construction site took a wrong turn and ended up at Los Alamos National Laboratory, where it triggered contamination alarms. By that point, the rebar had been used in a whole bunch of construction that had to get torn down.

[1] https://en.wikipedia.org/wiki/Goi%C3%A2nia_accident

[2] https://en.wikipedia.org/wiki/Acerinox_accident

[3] https://en.wikipedia.org/wiki/Ciudad_Ju%C3%A1rez_cobalt-60_c...