For reference, small portable devices containing Iridium-192 are frequently used for on-site NDT weld inspections. NDT inspectors often have a truck full of valuable equipment, they are frequently alone on the job site, and the welds being inspected are usually in remote locations. This is not a rare occurrence and odds are the thief has no idea how thoroughly unlucky they are.
So, it appears this type of incident is somewhat comment. Wikipedia [1] states:
"Iridium-192 has accounted for the majority of cases tracked by the U.S Nuclear Regulatory Commission in which radioactive materials have gone missing in quantities large enough to make a dirty bomb."
If you chase the wikipedia references, you get to this 2007 New Yorker article [2]. Since the New Yorker articles are notoriously long winded, here's the relevant paragraph:
"In the United States, between 1994 and 2005, the N.R.C. recorded sixty-one domestic cases of stolen or lost isotopes in amounts that would clearly be useful to someone making a dirty bomb, although the majority of these involved iridium-192, which loses its potency fairly quickly."
For reference, the half-life of Iridium-192 is 73 days.
In other words, the concise summary of this situation is "Move along, nothing to see here"
I found the frequency of contamination of scrap metal with radioisotopes to be disturbingly high (this article lists several more cases in addition to those in the wikipedia article: https://slate.com/technology/2013/05/asos-investigation-into...), and we don't seem to be able to reliably detect contaminated goods at the border.
Reading these articles inspired me to build a geiger counter to confirm that none of the metal items I own are contaminated. I used a kit (from https://sites.google.com/site/diygeigercounter/ , but other sites sell them as well) to build a detector that can reliably measure gamma for about $60. Tubes that can detect alpha (e.g. from contamination with Americium-241) as well as test sources (with comparable activity to an ionization smoke detector, i.e. safe and legal for anyone in the US to own) for both alpha and gamma are slightly more expensive but readily available.
https://www.ndt.com.au/product/qsa-sentinel-iridium-ir-192-s...
https://en.wikipedia.org/wiki/Industrial_radiography