Then at that point the signals should stop
shortly after because there is no point in
powering devices that are no longer working.
Think about it: if we find listening devices, disable them, and the ultrasound/microwaves abruptly stop this would strongly suggest that (1) we've found all the devices (2) the microwaves were indeed specifically for the listening devices (3) the planter of the listening devices is also the party responsible for the ultrasound/microwaves, which is significant because the number of parties that could have physically planted the devices is surely lower than the near-infinite number of parties that could be broadcasting the ultrasound/microwaves.
An adversary would surely want to keep us guessing, and would not want us to see such a clear correlation unless that too was an attempt to fool us. So they would likely want to continue with the ultrasound/microwaves even if they weren't getting a response from the listening devices.