iPhones have a feature where they still intermittently broadcast a Bluetooth signal after being turned off or even if they shut off because of low battery: https://discussions.apple.com/thread/255809396?sortBy=rank.

Google does the same thing with the Pixel: https://support.google.com/pixelphone/answer/9338817?hl=en-AU#zippy=%2Cstep-find-offline-devices-and-devices-without-power.

My understanding is that it's not the phone signal, but bluetooth low power mode, which is how many countries did covid exposure tracking.

So not full location triangulation, but more a hypothetical that a business or government could very easily set up bt low power beacons and identify devices that got in range like they did for covid tracking.

It's not cellular bands, I believe it's Bluetooth beacons and possibly UWB. But the fact that "Find My" works even when the phone is "off" is a pretty well documented fact. Of course if you could actually unplug the battery those would die too.