If they limit themselves to passive reception they won't leave much of an electronic 'footprint'.
That said, there's always *something* (google 'heterodyne') for some of the signal amplification techniques that might do that.
FWIW, I've been trying to dig through UNCLOS, and I can't find anywhere where it says that passive reception is forbidden - even in territorial waters needed for international navigation. The question is whether it violates 'innocent passage'. https://en.wikipedia.org/wiki/Innocent_passage