You're completely correct. You'd have to be watching that frequency continuously. Fortunately, there's another way...
Nonlinear junction detectors can find semiconductor things, be they powered on OR off. Long story short, you blanket an area with GHz rf, and then look at the harmonics of the freq you spray it with.
I can see how to create one with a 2.4GHz transmitter and a DSP. I know the prices Ive seen are in the thousands of $$$, in which it's not terribly complex. The hardware would probably cost around a few hundred, primarily cause DSPs are $$$$
