Been having this issue since Software 12 release. I read one person's response where they did three things at once but one of them hit onto something for me. I just Hike+ 2 when I go for a walk, but I don't go on one every day. But on the days after using it, I would see the battery life drain. So my thought was that the GPS was continuing to be on even though I had exited the app. So after doing on a walk and closing out the app, I turned off my watch, turned it back on, and the battery drainage was normal.
So I'm guessing something in the GPS part of the software is screwed up and it doesn't turn off automatically when exiting a app.
Your mileage may vary but that seems to be the solution that works for me. None of this "connect to computer, sync, power off by holding button for eons, then let connection turn it back off".