That certainly sounds much more plausible. Perhaps their level of gain is somewhere between enough microwave intensity (and flux) that a human body would have a noticeable sensation of heating, vs. enough microwave intensity and flux to seriously damage the brain. Something tells me that it's not a particularly well-characterized parameter space, certainly at least not in the public record. Random thoughts because this is fun: To effectively transmit something of a given wavelength, you need something very near the size of the wavelength (I know you know, 'bl00, this is for posterity). So if they targeted the second harmonic of brain resonance, they could get to masin' with using something ~2.5 cm for an antenna half of the wavelength. Small enough to hide, but there might still be issues related to power dissipation. It would be interesting to measure a sample of ceiling, wall, and floor material and check the skin depth across the microwave range. Or maybe they've found a polarization technique that is especially suited for brain scrambling? Regardless, I'll never be a diplomat anyway.
Let's continue the thought experiment. Wifi is 2.4gHz and around 100mW. Suppose they were boosting that at 10GHz to steal it? Cell phones are in the watt range; the shear amount of RFI you've got spitting around is going to make monitoring tough. And what if you PWM your wave and duty cycle it under 50%? If you're building hysteresis into your passive device, you're good to go and the average sweep probably won't pick it up. And you can jack it. It's really the cavity volume that will give you your resonance. The size is whatever. 2.5cm diameter? Fine, I'll make something the thickness of a playing card, feather the edges and put it under the paint at the tape and mud stage. I can hide an inch anywhere especially if I don't have to feed it power or pull off signal. If the duty cycle is under 100%, living tissue may easily act differently than anything else. And your own testing might not even reveal what those effects are unless you bathe convicts in it for a year at a time.