The way it works is that the Bluetooth and GPS stacks are powered on every few seconds briefly, even if the device is supposedly off.

iPhones that have "home" buttons demonstrates this not-true-sleep very well. The button looks like it is embedded in the front glass very well with a nice satisfying click when it's depressed...

...except that it's not a button at all. It's just a polished circular indent in the glass! There is no moving part.

The click? That's haptic feedback from the vibration motor that works even if the phone is "off". If the phone loses all power or is truly turned off, then suddenly the button goes dead and you realise that it's just smooth glass. I had that phone for years before I first noticed this.