Definitely not the most obvious location. I would have expected to find this under the bluetooth settings.
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)
I have a "store mode" button that just kills wifi/bt that I hit before I go into any store.