The only mildly strange thing about it is that such a check should take like 10 seconds and this article was apparently written after the capsule was found, but before the result of the check was announced.
But the serial number check itself is an obvious thing to do and it doesn't necessarily imply they believe there are more of the capsules missing.
It's going to take a bit longer than 10 secobds to safely get close enough to such a strong radiation source as to take a clear picture or read a number.
It's not that strong of a source. You wouldn't want to go about your day with it in your pocket, but last I read a 1m exposure for an hour was a few x-rays worth of radiation. Take out your phone and zoom in.
And since they knew they were looking for radioactive material, I would hazard a guess that they already have necessary protection.
The article was updated to say the serial number was confirmed, it took about three hours after the news first broke. Seems fair, probably took that long for the Australian Defense Force to get to the capsule.
Because the output pulse from a Geiger–Müller tube is always of the same magnitude (regardless of the energy of the incident radiation), the tube cannot differentiate between radiation types.
No "signature" detection possible with simple counters I'm afraid.
They used trucks on the road with large doped sodium iodide crystal pack (tubes with a scintallation (flash) detector at the ends) that produce a second by second full spectrum of gamm energies seen.
Processing software is used to filter out cosmic radiation signatures, the ground vehicle signatures, the mean expected background signature of the Western Australian region, and to enhance the target peaks from the Cs-137 source.
You'd think that was quick, ... but generally this kind of thing would be done from a crop duster airframe flying at an industry standard 70m/sec (252 km/hr).
I'm guessing the local Perth geophysical survey companies that routinely fly magnetics and radiometrics were all fully engaged flying pre booked contract work .. so they dragged in a couple of white transit vans and fitted them out to get the job done.
Seems straightforward enough in the geophysical instrumentation domain.
Exotic? Radiometric mapping has been around for 50+ years - Australia has mapped the entire country (size of mainland contiguous USofA) from aircraft with ~200m line spacing in that time (along with surveying other countries, Mali, Fiji, India|Pakistan border, elsewhere), Russia, South Africa, Finland, USofA also have radiometric survey teams.
It's handy for finding drums of radioactive waste in a Finnish forrest near the Russia border, for example, which was an actual contest | exercise some years ago.
Australian civilian radiation services with backing of AGSO (Australian Geological Survey Organisation) are easily able to handle this all within Australia w/out reaching out to the Indians, Pakistani's, Iranians, Russians, South Africans, or other international nuclear agencies.
So they're not certain they have found it, yet.
They could have found one of the other radioactive capsules lying about the Western Australia outback.