With iOS 15, your iPhone is still traceable through the Find My network even when the device is powered off. It seems that with iOS 15, the phone is not really fully ‘powered off’, it stays in a low-power state and acts like an AirTag, allowing any nearby iOS device to pick up the Bluetooth signal and send back its location.
In fact, Apple says the location tracking will even keep working whilst the phone is reset to factory settings with Activation Lock enabled.
Apple informs users about this new behaviour with an alert dialog when the newly-updated iOS 15 iPhone is running low on power for the first time.
Interestingly, the copy for the alert dialog references this feature to help recover stolen items. Usually, Apple does not advocate using Find My features to fight against theft.
If you want to make your iPhone actually turn off, you can change the behavior in the Settings and disable the low-power Find My mode completely.
FTC: We use income earning auto affiliate links. More.