I used to fervently keep my bluetooth off on iOS, and I learned that if you turn it off via the Control Center, then it automatically gets turned back on the next day. But if you turn it off via Settings, then it only gets turned back on when the system software updates. (I stopped doing this a couple iOS versions ago, though, so it may have changed since then.)
Bluetooth (and wifi) aren't turned off at all through the Control Center - they changed the wording to say "disconnected", meaning that your phone only disconnects from known devices. But both are still turned on for other purposes such as CarPlay, Handoff, and Location Services (via wifi). For the purposes of this discussion, they are potentially still transmitting a known identifier.
Apple reconnects to known devices and networks at 5am:
https://support.apple.com/en-us/102412
Bluetooth and Wi-Fi Aren't Fully Disabled When Off in iOS 11 Control Center
https://news.ycombinator.com/item?id=15297387 (2017, 143 comments)