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Not suggesting rad IS what they're looking for. But the element or isotope and any containment would play the largest factor in determining whether or not the material can be detected. Gamma, X-Ray, and Neutron energy can travel 100s of feet or more, and may only be blocked by dense material or material high in Hydrogen.
Generally, we get concerned with sensor spikes above 2x the naturally occurring background and use that as a starting point for identifying possible problems.
Ironically we use a Chinese DJI Matrice UAS with a RAE Systems Multi Rae (with Gamma detection) to do this very thing. Working with a "foreign partner" in 2022, they used backpack detectors and bicycles in order to navigate the narrow streets/alleys of that area as well as vehicle mounted systems. They really wished they had UAS capability in order to cover large, dense areas rapidly at low cost.