Back when the US was still interested in nuclear non-proliferation, they sponsored (together with the EU) the installation of radiation detection equipment at borders and other chokepoints, mostly in poorer countries.

These are fixed installations, monitoring mostly truck traffic 24/7, so their effectiveness doesn't depend on available manpower or current interest. Eastern Europe from Poland to the -stans has a lot of them, because former soviet republics had a lot of radioactive material lying around that was suddenly less important and even less guarded after the Cold War.

(Scientists were also suddenly out of work, and feared to be receptive to corruption. Fun fact: George Soros spent dozens of millions of his own money to empty a bunch of nuclear physicists in the 90s).

The point being: if it were a widespread problem, it would definitely be noticed.