Some relevant-looking thing I found on Google:
- "9. SEARCHING FOR RADIOACTIVE OBJECTS"
- "The first part of this section describes three different airborne surveys that successfully located lost radioactive sources. The three incidents were quite different in nature and provide useful information should similar accidents occur in the future. Section 9 concludes with a discussion showing how to estimate the count rate which would be observed by AGRS from a radioactive source. These results can be used to plan searches for lost radioactive sources."
- "9.1. RECOVERY OF A LOST 60CO SOURCE"
- "9.2. LOCATING THE LOST ATHENA MISSILE"
- "9.3. COSMOS-954 INCIDENT"
https://inis.iaea.org/collection/NCLCollectionStore/_Public/... ("Airborne Gamma Ray Spectrometer Surveying", 1991)
Their first incident is basically identical to this one:
- "On 21 June 1968, a 325 mCi 60Co source3 was lost in transit somewhere between Salt Lake City, Utah, and Kansas City, Missouri, a distance of 1800 km."