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)