Apple, Mac, MacOS, iOS4, iPad, iPhone and (in)security...

To be fair to Apple, this behaviour is officially documented in Apple support article HT208086, entitled Use Bluetooth and Wi-Fi in Control Center with iOS 11, but in our opinion, you’d be forgiven for thinking that those those grey “off” icons on the Control Centre screen really meant “off”.

As the support article explains it, “off” in this context means something much more nebulous, namely:

Both Wi-Fi and Bluetooth will continue to be available, so you can use these important features: AirDrop, AirPlay, Apple Pencil, Apple Watch, Continuity features, like Handoff and Instant Hotspot, Instant Hotspot and Location Services.
Curiously, the support article also documents that both Wi-Fi and Bluetooth will automatically come back on when one of these happens:

You walk or drive to a new location. (Wi-Fi only.)
It’s 05:00 local time.
You restart your device.