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.