In Rick Atkinson's "An Army at Dawn" detailing the US Army at war in North Africa, when he references the security preparations for the Casablanca Conference, he detailed Armed Guards, Mine Detectors, and Geiger Counters.
I know that the Manhattan Project had been going for some time by then, but outside of that I haven't seen any other reference in any other history of the period of any substantive awareness of Nuclear Physics. Today (and at any time since 1945) it makes sense for using Geiger Counters as part of a security sweep for an International Conference. I'm curious how widespread the practice/awareness of utility or reason for concern that would call for the use of Geiger Counters at such an event in 1943. It feels like an anachronism outside of the Manhattan Project and supporting efforts. Why would they have been doing so in North Africa in 1943?
There were US fears of possible use of nuclear reactor poisons by the Germans in World War II (basically as a "dirty bomb"), as I've written about here. Early 1943 seems pretty early for that, and I've never seen anything in the files about North Africa, but it's not a totally out-of-time idea; this was right around when they were starting to worry about such things.
Using a Geiger counter to detect a nuclear weapon, it should be noted, would be fairly insufficient: they're not that radioactive (and the nuclear parts are covered by lots of other things that block the radiation), and you'd sort of have to be right on top of one to detect it. (Even with modern technology, it is very difficult to detect enriched uranium, because its radiation is easily blocked.) In the postwar Oppenheimer joked that the best tool for detecting a smuggled atomic bomb would be a screwdriver — so you can open up any boxes and look inside them.
So my guess is that if this happened, they'd be looking for reactor byproducts, not a nuclear weapon.