Unless you can physically disconnect the battery, your phone may not actually be fully off.

For example, on my Android phone should I drain the battery into oblivion ( let the phone die ) and let it sit for about a week in that state, takes about 90 seconds to become fully functional to the lock screen from the moment it is plugged into a charger.

On the other hand if I do hard power off of a phone followed by powering it on, it takes ~35-40 seconds for a phone to get to the lock screen. Out of curiosity I tested several more handsets with similar results. I can only explain that difference by phone not being completely powered off when the battery is inserted unless it does not have any juice at all.

I had assumed phone hardware generally won't power on until the battery has charged to 5% or so, ostensibly to prevent power drops. Not sure which idea is more accurate without a mobile electronics engineer chiming in.

That is correct. The phone waits until it has enough power to ensure it won't die during startup because a lot of modern phones (and ultralight laptops) require tapping into battery reserves because the direct pathway from the wall wart to the device was not designed to provide sufficient power to operate at full speed/capacity for more than a brief turbo boost and startup requires more than that.

Battery charges "enough" to that point in less than a minute? I find it hard to believe. 4-5 minutes? Absolutely. 50 seconds?

I suppose the timing depends on if you have one of the anemic iphone ones or a full 2.4+ amp USB charger. Given that charging status is unreliable at low charges and charge tends to "come back" a bit when not actively drawn.

My current phone has an old battery and can go directly from 40% to 70% charge in one refresh cycle (10-30 secs?) after plugging in to my ikea charger.

It's called warm boot vs. cold boot (or soft boot vs hard boot)

Yeah. The big question is "Just how much of the device is 'warm' while waiting for a warm boot?" I _suspect_ many people here could make reasonable guesses about the warm/cold states of the OS rebooting, but I'm sure far fewer people have the required knowledge/experience to even reason credibly about the state of the cellular radios (and probably the GPS receiver) when a phone is "warm".

(I could, I suppose, sit a spare handset next to an SDR rx, and watch it for cellular transmissions as the phone sits in "presumed warm" state. But a negative result there would not convince me it isn't still in some sort of "active receive" state where it could be remotely woken up by sending it somesorty of Wake On LAN equivalent in the cellular radio world...)

You can use a non-linear junction detector, and triangulate on the signal it forces the target radio to give off. They are not very selective, so it’s easier to track when there are very few radios in the area under illumination.

The easier way is to compromise the phone and have it pretend to be off.

Does the first method work if the battery is removed?

Could the non-removable battery "feature" of newer phones be a result of governments requiring the manufacturers to do this?

Those don't pick up just radios, they pick up any transistor.

Your phone is never really off.

Heads up: your account has been autobanned, maybe because you're commenting too many times as a new user, or for some other reason.

How can you tell?

Turn on "showdead" in account settings. This will show comments from banned users.

I am only guessing they were autobanned. Comment history from the time of the banning doesn't show anything particular egregious that would trigger a manual ban.