That’s not quite how it works, though. These devices are basically mini computers now, there’s a limit to what they can do without fully booting. Devices that are plugged into the wall might be likely to retain some power-draining function while plugged in, but there’s only so much you can do on a trickle charge while a phone is powered 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.
