My first thought is that many if not all modern CPUs have a separate master CPU within it (such as Intel's Management Engine) that can run even when the PC is turned off.
It runs when your computer is asleep, while it’s booting up, and while your operating system is running. It has full access to your system hardware, including your system memory, the contents of your display, keyboard input, and even the network. https://www.howtogeek.com/334013/intel-management-engine-explained-the-tiny-computer-inside-your-cpu/
Could this be what is happening?
It makes me wonder, with all the legislation being drafting in Western nations, why no privacy laws stating manufacturers can't put embedded corporate spy devices in the devices they sell you? Strange isn't it.
Shutoff is a multiple different things now. Often, when you shutdown laptop by button or from win/ios, it does not turn off the power, but fall into suspend mode, where everything except RAM is turned off to keep the contents of the RAM intact and immidiately continue from the place you stopped when you turn it on. But RAM used in modern PCs is dynamic one and eat noticeable amount of power. And things goes worse with every new dynamic RAM generation.
If modern OS sheduled applying updates, it will not shutdown really, it will just turn off screen and continue to work installing updates. This could eat battery very fast, since for unknown reason applying updates in win/ios is not just unpacking new binaries in place of old ones as in normal systems, but a lot of power hungry searches, rebuilding databases and tons of other stupid stuff due to stupid OS design.
I met some laptops that had strange vendor software, something like "WhateverBrand Laptop Control Center" with "battery care" folder/section with option of "thorough battery check" - it tries to discharge battery for some time loading processor with useless calculations and calculate battery real current capacity from voltage drop under serious load. As usual for "vendor" soft such utilities are low quality and often have serious bugs. So, if "periodic thorough bettery check" option is turned on, it could occasionally endlessly loop in that discharging test. Always remove any "vendor helper utilities" from your computer if you are not ready to investigate sudden strange accidents.