If the signal is strong enough, you can easily design a pocket detector. A PCB attached to a pocket protector with antenna tracings for multiple spectrums and a buzzer. No need to get fancy, a few passives off the shelf will do the trick. Or just get a SDR scanning multiple spectrums and have a threshold trigger above a specific power level. This is beginner radio stuff, I am sure their local hardware tech support can whip one up under a day.

I wonder if you could just have a calorimeter and a block of material with similar properties to human tissue? That way you cover the EHF/THF(>30GHz) ranges as well?

I guess semiconductors are covering pretty high frequencies these days though(http://www.semiconductor-today.com/news_items/2019/may/knu_2...).

Terahertz doesn't penetrate walls particularly well but if they managed to get it to work, then that would be very interesting.