> He said a search vehicle was driving past at 70 kilometres per hour on the Great Northern Highway when a detection device revealed radiation.

They found it because it is literally a beacon.

I think the idea would be to have a gieger counter in the vehicle near the pellet that can detect the pellet radiation, and sound an alarm in the vehicle if the pellet radiation is no longer detected, so the driver knows immediately that the pellet has become unsecured.

They found it because they got incredibly lucky.

If it would have bounced just a few meters further off the road, landed in a ditch and get covered by something, or its emission beam angled away from the road/sky (for directional sources), it might never be found (or not before it causes some harm).