Wi-Fi turned off but battery usage shows shows huge drain from it
This is a common misconception with Android. When you turn off Wi-Fi, Wi-Fi does not actually turn off. There is an advanced settings menu where you will find an option that either says Wi-Fi scanning or Wi-Fi always on. Google uses Wi-Fi points to determine the device's location without activating the GPS.

Turn Wi-Fi scanning off to fix this. You might want to turn off Wi-Fi while asleep while your at it. It's in the same menu.

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Edit: In Android 6.0 (Marshmallow), this option has been moved. It is now located in Location > (Overflow menu) > Scanning (Thanks, nanny)