Spying with microwaves is actually reasonably easy, because electromagnetic frequencies reflect physical vibrations – they become a carrier for sound. So even without a microphone, EMF signals can under the right circumstances contain the traces of sound waves as amplitude modulation. s
Now, this won’t come as news to anyone who’s a fan of the history of Leon Theremin, because the inventor of the instrument of the same name also pioneered the technique. I’ve actually shown his invention (The Bug) in lectures, because the story is just too good.
But I think what’s more interesting than any of this is the fact that spying is all about doing something that the other person doesn’t expect. That’s what made Theremin a genius. He took a technique that was technologically simple and applied it to a new context, in such a way that no one had thought to look.