[hwillis]

White matter is the deep part of the brain. Microwaves wouldn't penetrate more than a couple cm.

Personally I think something much more benign is probably the answer. Maybe someone introduced drugs into their food/water? Sometimes a noise is just a noise.

[QAPereo]

I don't know, if this is some kind of sophisticated multiple-beam system using constructive interference (https://xkcd.com/1922/), it might have more penetrative ability than you'd assume.