Here's a more technical/curiosity-satisfying answer to the question the title poses, https://inis.iaea.org/collection/NCLCollectionStore/_Public/22/072/2207211

Here's a more technical/curiosity-satisfying answer to the question the title poses, https://inis.iaea.org/collection/NCLCollectionStore/_Public/22/072/22072114.pdf https://inis.iaea.org/collection/NCLCollectionStore/_Public/... ("Airborne Gamma Ray Spectrometer Surveying", 1991; 116p) There's some really interesting case studies in chapter 9 ("Searching for Radioactive Objects"). The first one was more or less identical to the highway incident in Australia: "On 21 June 1968, a 325 mCi 60Co source was lost in transit somewhere between Salt Lake City, Utah, and Kansas City, Missouri, a distance of 1800 km." The other two were an American test ICBM that crashed in remote desert in Mexico, and a Soviet orbital nuclear reactor that crashed in Canada.