At Sue Schmidt’s gas station and roadhouse off a remote highway in the Australian Outback, employees usually watch out for snakes when they are walking outside. But this week, they were looking for something else: A tiny capsule of radioactive material that sparked a search along a roughly 900-mile stretch of the road.

The capsule, used in mine equipment, went missing while in transit from a Rio Tinto PLC mine to Perth, Western Australia’s state capital. As the search dragged on over the past week, Ms. Schmidt and her employees grew wary of cleaning up the bottle caps and coins that they usually find outside the roadhouse, fearing that any shiny object could be the capsule that would hit them with a dangerous dose of radiation.