Pattern Recognition Anon Chimes in on Drones
Going off this theory (drones as nuke sniffers), I think looking at radiation maps, train down detectors, ship/marine tracker apps, and flight radar would give us a ton of info. Also consider the ports our enemies might try and use. Large ports probably have the capacity to detect something bad coming in, while smaller more out of the way ports could be more vulnerable and lack detection methods. I'd look around Florida, Louisiana and the Mississippi river, maybe Texas. Look at the registration for vehicles/planes, and consider that they would probably try and use the same smuggling methods that human traffickers and drug traffickers have used for years now. You could even consider that those techniques may have been dry runs for this exact scenario. Mexico's president was talking a ton of shit lately, and isn't happy about the tariffs, maybe just like in WW2 a foreign power has offered them a deal. Bad guys might even try cloaking the bad material by keeping it near something that has a radiological signature such as a hospital or power plant simply to hide it from sniffer drones or on a truck that claims to be transporting medical machinery. Houseboat moving fast up Mississippi, radiological food pasteurization machinery (we have sure had a lot of food recalls lately, haven't we?), what else might a sniffer generally write off that could be used as cover?
As an aside, I would speculate that the REAL Project Blue Book program is exactly what we're seeing. When the sniffer drones are up, just call them aliens. Last thing we need is the stock market tanking or general chaos.