https://motherboard.vice.com/en_us/article/evpz7a/turn-off-wi-fi-and-bluetooth-apple-ios-11

Turning off Bluetooth and Wi-Fi when you're not using them on your smartphone has long been standard, common sense, advice. Unfortunately, with the iPhone's new operating system iOS 11, turning them off is not as easy as it used to be.

Now, when you toggle Bluetooth and Wi-Fi off from the iPhone's Control Center—the somewhat confusing menu that appears when you swipe up from the bottom of the phone—it actually doesn't completely turn them off. While that might sound like a bug, that's actually what Apple intended in the new operating system. But security researchers warn that users might not realize this and, as a consequence, could leave Bluetooth and Wi-Fi on without noticing.