We carry “radioactive material” for several reasons. Radiation sources are included in some equipment for chemical/biological/nuclear weapons detection, equipment calibration, medical equipment and treatment, etc. I used to have to personally sight some of these sources monthly. The word about these materials is put out to warn responders and lookyloos not to mess with the crash site, as there could be an exposure risk.
Yes, most likely a relatively weak sealed rad source for detector calibrations and daily QC checks, and not likely to be much to worry about.
I couldn’t see any reason for them to have something like a radiography source on an Osprey.