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.