Was wondering if anyone remembered the sniffer planes. Been outta the game for 20+ now but they were used for all kinds of things. Development of new systems and detection tech never stopped while I was there. City level stuff was mostly chem/bio, can find meth labs that way very easily. Adaptable to "other" chem processes. Guarantee that most of what we had at the time has been generationally superceded probably at least twice since then. If they were flying downwind tracks they were probably looking for chem/bio markers. Narrow it down, then can find it via ground vehicles disguised any way you want. Moving trucks are easiest though I doubt the equipment needed is THAT large anymore. Minivan or SUV driving around draws no real attention. Drone mounted would be an option for close in localization with newest miniaturized gear. So many options with new tech.

Detection for nukes ... whole different ball game.