If the iPhone has iOS 15 installed, the phone will not truly be turned off and instead, stay in a low-power state and acts like an AirTag. This mode will allow other nearby iOS devices to pick up its Bluetooth signal and send back its location to the owner.
Apple says that the new tracking method will also let users locate iPhones if their device has been reset to factory settings. The iPhone will have the Activation Lock enabled and Apple will inform the user about the phone being reset with an alert dialogue. The text in the alert refers to a feature that will help in recovering stolen items.
See iOS 15 Will Let iPhone Users Find Their Phone Even If It’s Turned Off Or Has Been Reset By Thieves
Losing your iPhone is getting more & more difficult.