[pcl]

I've heard that theory a number of times. I'm no expert in acoustics or eavesdropping, but I would assume that ultrasonic eavesdropping equipment would involve highly sensitive microphones, not powerful speakers.

Is there some sort of ultrasonic-carrier-signal microphone out there, akin to laser mics?

[arca_vorago]

Think of a bat. It clicks and the reverb is what its ears interpret to understand surroundings. Output can be used to trigger/power passive listening devices. I always think of the Russian gift to the US embassy that was a wooden seal with a passive bug in it. Scans didn't catch it for years.

[wavefunction]

That was activated by and transmitted radiowaves though.

[KGIII]

Hmm... A speaker is a microphone. I am not sure if that is salient, but it seems worth noting.