I really don't follow this narrative. Yes it's hot and remote and it's a 1400 km road. All of those things are clear. It's still very easy to put sensitive detectors inside a vehicle and drive along the road. These are uncommon and fairly expensive devices but there are plenty of them around. If the source is on or near the road it will be found that way pretty easily.
The road is mostly sealed (Perth (State Capital City) -> Mt Newman (Big hub town in Pilbarra)) with a short tail end road of maybe 50 km to the site in question that'll be probably graded gravel - rough, bumpy, but essentially flat.
With five days gone it's already been flown and driven .. at this stage they're either looking at trace signal and indications the slug has gone well off route in a truck tyre OR doubling back to see if any site workers flying out pilfered it as a souvenir and flew out to Thailand for a holiday since.
