The solution to the battery drain issue is nothing to do with pairing and unpairing. A few weeks back my Watch did the same thing. At 10% battery it notifies me, and even after rebooting it immediately it still died within an hour of reboot. Same thing happened to my wife a few days ago while we were watching a movie. 10% battery and then dead soon after.
Both of us usually have 50-60% every day before we go to sleep.
The solution is not to reboot but to put the Watch on the charger. You don't even need to reboot. I don't know why this is happening, but I'd be more inclined to believe it's an OS or Apple app but because neither my wife or I have any third party apps that we use except Uber, and that hasn't been used in a while anyway.
Anyway, this is just our experience. There's no need to unpair the Watch when this happens. It'll just happen every now and then and there's not much any of us can do about it.