The answer might surprise you

Here’ what is known as of yesterday !

I don’t know, but there are several theories out there that experts can debate.

Havana Syndrome (book)
It is one of the most extraordinary cases in the history of science: the mating calls of insects were mistaken for a “sonic weapon” that led to a major diplomatic row. Since August 2017, the world media has been absorbed in the “attack” on diplomats from the American and Canadian Embassies in Cuba. While physicians treating victims have described it as a novel and perplexing condition that involves an array of complaints including brain damage, the authors present compelling evidence that mass psychogenic illness was the cause of “Havana Syndrome.”

Authors
Robert W. Baloh, MD is a distinguished professor of Neurology and Head and Neck Surgery at the David Geffen School of Medicine, UCLA. Author of 11 books and over 300 articles in peer-reviewed science journals, he is a pioneer in the study of the vestibular system: the part of the inner ear which helps people to maintain their sense of balance and spatial awareness. He has developed tests of vestibular function that are used by inner ear specialists around the world.

Robert E. Bartholomew, PhD is an Honorary Senior Lecturer in the Department of Psychological Medicine at the University of Auckland. He completed a doctorate in Medical Sociology from James Cook University in Australia and his Master’s in Sociology from the State University of New York at Albany. He has published in over 60 peer-reviewed journals, has been featured in a National Geographic series on modern myths and has appeared on The History and Discovery Channels. A Fellow with the Center for Inquiry in Amherst, New York, he teaches History at Botany College in Auckland.

I know nothing more than what is public.
But, I can imagine how a device used to surreptitiously monitor the inside of a building would be emitting microwaves to penetrate the walls. Since the brain is an almost perfect absorber and potential antenna it is conceivable that it would absorb some of that energy and cause a reaction somewhere.
If they are doing it from a distance the beam would be high power and directional.
A special broadband receiving and recording station should find this type of radiation.

There are examples of using a “casually discarded” potato chip bag moving due to acoustic vibrations in a room. An IR laser is aimed at the bag and the reflections are demodulated to hear the speech. This can be performed through the window glass from great distances – across the street in another building or with a station located a km away.
It is even possible to bounce the laser off the window glass itself.

I just listened to this podcast earlier this month: https://podcasts.apple.com/nz/podcast/havana-syndrome-qu%C3%A9-diablos/id278981407?i=1000515957128

Wow.

No plausible cause in known science. Dramatic clinical results, rejected by scientific bodies for lack of standards. A nebula of fairly common symptoms. Sufferers are all workers in high stress, close knit environments. Everything about this syndrome screams “psychogenic” to me.