Ever since downloading IOS 12 on my IPad Pro, my battery started dying overnight. Found info on line that suggested this and that caused it, Siri and voice commands etc, followed this advice and still it would die. Battery usage did not show anything.
So, one morning I had realized my 920 was on the charger all night and my iPad battery had died. Then some lightbulbs started coming on. 920 has wifi. It was talking to my iPad and garmin express all night. Just shutting off the watch did not entirely solve the issue. Found out my desktop computer had Garmin express running in the background, found it while I was ejecting a thumb drive. It (desktop, which also has Bluetooth and wifi) was also talking to my iPad. Once I disconnected the express icon my problem started going away. So now my iPad retains back to normal battery usage. This issue does not affect my iPhone 8. Hope this helps. I’ve also started turning off the iPad wifi and Bluetooth when I go to bed, just in case.