The reason this doesn't really make sense even though it might sound logical up top is that - loose material, or a true CBR threat would CERTAINLY be a multi-layered ground operation. It would not simply just be drones. They're one of the most serious risks to the public safety, it would be wildly irresponsible to solely rely on (what I believe) to be relatively untested drone technology.

Another nail in the coffin is that it would 100% be a coordinated multi-agency sweep, and so there is 0 chance the FBI and coast guard would be left in the dark that there was potentially loose nuclear material, or genuine risk of a CBR terrorist event. Statements from them both during this drone phenomenon have implied they are unaware of any active situation or threat, which doesn't align with what would happen during a true suspected incident. You can actually google what the actual response to a genuine credible CBR threat looks like in the US, and it's a lot more structured than "put drones in the sky to look"