Did you read the article? There were a couple cases were very early Android phones were modified to appear to be off but stayed on. This is fairly common knowledge, but it’s not particularly hard to defeat.
Everything your phone does requires a deterministic amount of power. Spying on people in particular requires even more power than normal because you need to run the power hungry gps in addition to the modem and cpu.
If you turn off the device it should be significantly cooler to the touch, not a degree above ambient. If it’s at 100% charge but a power bank with a read out is showing it still charging, that’s a problem. Is the bootloader image different? You can verify that to some extent. When you turn it back on has it been drawing down the battery anyway? Does it require an unlock password instead of biometrics as it normally would (assuming a particularly sloppy setup)?
This isn’t rocket surgery, in reality nobody is modding everyone’s phone to stay on forever because unless you’re an absolute troglodyte (aka the fucking old school mafia bosses they did this to) it’s going to be painfully obvious your phone is acting weird.
Depending on the chipset you can usually set rtc wakeup timers, though that typically implies sleep rather than power off so you’d still have some power draw when the device should be off. Similarly, if you’re trying to log GPS you’ll have to wake up for enough time to get a GPS lock so even at something like a 10 minute logging interval you’d get some noticable power consumption. Much much more if you’re trying to log voice or video.