I've noticed that my watch will suddenly drain its battery after acting normally all day. It's about a year and a half old, and I don't use the always on feature, and I use it to track workouts maybe 3 or 4 times per week.
Mine will be normal for random periods of time, sometimes most of the day, sometimes just an hour, then it will practically bottom out. Rebooting 2 or 3 times a day seems to help, but is tedious.
See my post over on this other thread if you are looking for a work around (disable notifications from Gear app). https://eu.community.samsung.com/t5/wearables/galaxy-watch-disconnecting-and-draining-battery/m-p/20...
It definitely looks like there has been a software update to the Gear app that is causing these problems.