Spectrum analyzers?
>Spectrum analyzers of the kind Balakaev is accused of smuggling are often used to detect radio signals to identify hidden surveillance devices, while the signal generators are often used to securely transmit information as a part of covert operations.
Those might be the portable devices, the sort that you can get off Alibaba for $75. Except that you'd be better off getting them from a US source because there are numerous variants, some of dubious quality. Bench devices are rather more expensive but the sort that might be used for building, testing and repairing 'surveillance devices' are relatively cheap these days, especially on the used market. Amazon sell a bunch of different units.
> Altair 4X gas detector,
A portable device used for mine safety, etc. Can be bought through Amazon.
I'd guess this was less about 'clandestine exports to the FSB' and more about 'ordering stuff for a friend without realizing that you've just become fresh meat for some ultra hungry enforcement operation. Its the sort of mistake anyone could make (including me -- it would never occur to me that commodity technology, a lot of it "Made in China" anyway, would be on the US's banned list, much less that there were bureaucrats tracking this stuff on my dime (I'm a US taxpayer).)