If you're willing to believe anything can be put in passive mode, you'll absolutely drive yourself mad. There's no way to proove that a device is not listening passively, so of course people will say it is.

Anything with an integrated circuit can be proven to be not operating based on power consumption or switching noise of the transistors in the device.

Modern microphones need input clock to operate, and are physically incapable of listening when the clock is absent.

This gives a trivial way to detect of the device is listening passively, at least on the workbench.

I don't even know what the "mic needs a clock" means. I've never used a microphone that needed a clock. Making a mic that needs a clock just seems like a "WTF were they thinking" kind of thing. Can you elaborate on this concept of requiring a clock for a mic?

The keyword to search for is "digital mems", and of course we are talking individual parts to place on PCB, not the consumer device. Here is one random example: [0]. Somewhat expensive at $1 ea, but that's what you pay for US manufacturer.

This microphones take 2.4 MHz clock and return PDM (pulse density modulation) signal. Does not seem very convinient for old-school analog electronic circuits, but it's pretty trivial to interface with any micro with pulse counter hardware.

And presumably the audio won't have any hum or interference from nearby radio transmitters, which is especially useful in cell phones.

[0] https://www.digikey.com/en/products/detail/cui-devices/CMM-4...

My phone has hardware kill switches for modem, wifi/Bluetooth and mic/camera.

Do those meters work across all frequencies?