> The unsettling part is that location logging continues to work with the phone off

There are specific bits of malware that make the phone appear to be off when it is in fact powered on. Such malware typically requires root access or an exploit, and is the realm of shady governments not advertiser's. But if you don't have that malware, you can't be tracked when the phone is off.

I'm sure there's plenty of RF energy to make a passive phone case that lights up when your phone is transmitting, which would detect this case if you were looking at your phone. Maybe having it emit a tone would be better.

It would be fun to see if you could get a modern phone to shut up for even five seconds when powered on.

> location logging continues to work with the phone off

I’m very skeptical about this. I’d be interested to know how a phone could track location info while powered off.

If you actually turn off location services in the OS none of this tracking occurs. About the only remaining source would be the mag of apps implement their own harvesting of the Earth’s magnetic field. Such things are extremely technically difficult to do well (you’d need to map the mag signals across the world) and the accuracy isn’t great (somewhere between wifi and coarse cellular if I recall correctly).

If at any point this actually became a reality the OS would start treating mag access as needing location permissions.

> If you actually turn off location services in the OS none of this tracking occurs.

Location services doesn't disable bluetooth. If you're in a dense area, your local stores may have bluetooth beacons which will report location hits against your bluetooth ID, which may be tied to you through applications you install.

https://www.nytimes.com/interactive/2019/06/14/opinion/bluet...

You also need to turn off the Bluetooth, cell, and wifi modems, and keep them off.

https://www.zdnet.com/article/us-cell-carriers-selling-acces...

It wouldn’t surprise me at all if baseband processors eventually start logging and asynchronously sending gps data to the cell carriers behind the operating system’s back, so you should also keep it in a mylar sack.

Similarly the accelerometer can gather all sorts of information (passwords, text typed on the keyboard, are you driving, flying, walking, etc), so you’ll probably eventually need to keep the phone in a drawer to avoid leaking that information. Similarly for the microphone (some surveillance libraries already snoop sounds in various primitive ways).

To recap: keep it off the network in a stationary, electromagnetically shielded, sound proof box. That’s simple enough, and from there you can use it as normal.