Stop Using Your Face or Thumb to Unlock Your Phone
The laws surrounding 5th Amendment protections and biometric passwords are still undecided, so just turn it off.
They're still running in low power mode and can wakeup from the network so they can absolutelly be made to "boot up" without turning the screen on and you being aware of it.
This is not like a bloody PC were the lights turn on and you can hear the fans when the thing starts, it's a machine with a low power mode in which it can already do a lot and which can be brought to a high power mode if needed without there being any visible or audible side-effects to alert the user.
Unless you completelly cut it off from power (by taking the battery out, which you can't in most modern smartphones) that smartphone with the lights off, the screen off and making no sound at all can just as easilly be in low power mode waiting for you to press the On button, as it can be in full power mode with a mobile network connection active, accessing the microphone and the GPS microchip and sending that data out, and both will look exactly the same from the outside.
I think you are overestimating what these devices can do when turned off, specifically when whoever is doing the tracking wants to be covert. Devices like Cellular Radios and GPS chipsets are getting more efficient every year, but they still consume enough power that it would be noticed if they came on by themselves even if the device was off.
I have an EE degree and have actually done work with embedded systems, including GPS.
The peak consumption of things like GPS is maybe 100 milliamps, with the average being in the tens of milliamps.
The wireless networking stuff is similarly frugal.
Further, stuff like encoding of audio is all done on the hardware and very efficient so even voice capture and encoding to send over the network isn't processor intensive.
Further, the CPUs on those things are ARM designs or equivalent, specifically crafted for low consumption and which have tons of tricks to avoid spending even a mW extra of power if it's not needed (basically the CPU will tend to activate only the bits it needs and use only the resources it needs to accomplish the operations its running, so it's almost never running at peak consumption).
The really big power consumption in modern smartphones is the screen and from very high GPU/CPU usage in things like games.
I think you seriously overestimate the similarity between modern portable devices design to operate from quite small batteries and things like desktop Personal Computers which are designed to operate from mains power.
If all they're doing is sending your GPS position out over the netweork every couple of minutes you won't notice that the battery has drained a tiny bit faster than expected even if you keep a keen eye on consumption because so little power is used to run just that part of the functionality.
Doesn't a modern smartphone have something like a 4000 mAH battery? And that lasts most people all day with room to spare? Even 100 mA every few minutes will get noticed, if someone has their phone off and expecting consumption to stay minimal.
And that's the key thing here, you're not just building a tracking platform but you are building it into commodity phone hardware without the users consent, and without them noticing. Any phone that burns that much power while off would likely get replaced by the user. Do you think the phone vendors are in on it?
It's not 100mA every few minutes, it's 100mA when calibrating from scratch with no satellites known.
I looked it up and the consumption when in normal use is around 30mA, which would mean that, say, if it took 10 seconds (probably a lot more than needed if you're not travelling) every 5 minutes - which adds up to 120 seconds @ 30mA per hour - that would consume 1mA/h (PS: by pure absolute chance my numbers ended yielding a result of 1 ;)), which is 0.025% of that battery per hour. If you're lucky, in the phone screen were one would be visualizing the graph for the battery power charge over time that would make the line fall 1 pixel.
It really is a whole other world out there in the embedded and low power systems domain.
In order to not "start from scratch", though, you will need to save some state persistently about your location (and the location of the satellites), which will cost power. Then you go in a building and lose all your signal, while still burning power to maintain that old state.
If it was that easy and cheap in terms of power, AirTags would have GPS receivers. They don't.
Flash memory preserves data without using any power at all. Ditto EEPROM. Both present in even the most basic of embedded processing cores (and the GPS protocol is implemented on those)
You need to move quite the distance for a GPS device to need to change just one satellite, much less all 3 and it doesn't matter if you've been underground or not as the thing will just try first the ones in its memory and unless you travelled hundreds of km underground, it's still going to be the same 3 satellites.
Last but not least, AirTags use CR2032 batteries with a capacity of around 200mA/h - 1/20th of a mobile phone one - and that charge is supposed to last for years between battery changes, not a mere few days until the next time the phone is charged. The power consumption of an AirTag must be thousands or even tens of thousands of times lower than what we've been talking about, in the order of nano-amperes not tens of milliamperes.
You're clearly clinging on to that pre-conception of yours for reasons other than logic, and you keep on inventing wild theories based on zero domain knowledge, to try and justify that beloved pre-conception of your, so I'll leave you to it since this feels like trying to explain that the Earth is roughly spherical to a Flat Earth believer.
You seem to be the one going through mental gymastics to justify why the button might not just turn the thing off. Sometimes they're not out to get you, you know.
These phones cram oodles of stuff into a tiny space at super low margins , and are perfectly good at spying on their users when turned on. There's no reason for them to spend any extra effort to spy when they're turned off, for the .01% of people who turn their phones off regularly.
The margins aren't as low as I thought, but they still aren't giving any money away on their BOMs....
Let me explain this in a very very simple way: buttons which are not literally mechanical switches that physically connect and disconnect from power require that at least some of the circuitry to be alive because they're capacitive contacts, a technique which requires some power and some logic to detect that the button has been pushed.
So even shitty shit $0.12 microcontrollers often come with support this stuff, so that they can generate a hardware interrupt in the microcontroller to wake it up when a user presses one such soft button to power on a device.
Beyond this, in order to support something as simple as wakeup from the network side - for example, to support Find My Phone functionality - even $3 microcontrollers (not microprocessors, microcontrollers, their cheap cousins with puny computing power) have features such as programmable secondary low power cores that consume tiny amounts of power.
Even this "advanced" stuff doesn't add cents to BOMs, it only adds tiny amount of extra surface on vastly more complex microchips, which translates to at most tenths of a cent of extra cost because this stuff isn't supposed to be decoding videos or running some social media user interface (or any user interface), it's just running small simple programs which might use a few peripherals configured to remain active in low power mode (and those can be network related) to listen for certain conditions and decide if it should wake the main cores up or not.
The functionality isn't there in the hardware because they added it to facilitate spying, it's there because that's just the direction the technology evolved in the last 2 decades - soft buttons instead of mechanical ones, some amount of always on functionality for fast start, support for convenience features for users, that require some kind of wake up from the network side or merelly because microprocessor or SoC makers add everything and the kitchen sink to their designs to try an make that chip usefull for the broadest list of use cased possible (it's quite insane the amount of stuff built-in in even the cheaper of the the current generation of SoCs) so that those chips are used in more devices and get sold more.
But it gets better: none of this is necessary:
Now, maybe somebody who has never been involved in Politics, or Demonstrations, or Strikes can go around with total confidence that their phone ins't hacked, but if you're anywhere close to the organisers of the kind of public demonstration that can snowball into to the current POTUS losing an election, don't assume your phone hasn't been hacked (which can be done remotelly) and that turning it off in the soft button marked power when you go into a meeting with other organisers has actually in fact fully turned it off in a way that makes sure it isn't spying on that meeting.
