I had the exact same issue on a 6 months old watch!
My battery life was around 2 days as with most say with a moderate use of occasional GPS, few sports apps beside full time connection.
then came one dreadful morning I woke up to a dead battery. Not suspecting much of it, I recharged again only to realize after 5-7 hours that battery evaporates (15-18% per hour). Unlike you, I had no update, did not change no setting, nothing - problem came out of nowhere.
The weird thing was that It did not matter If I turned on Power Saving mode or reset n number of times. All the same, battery gone in a few hours. Seeing that and similar issues online, I realized this is not something can be resolved.
I was really pissed such a silly fault product can make its way to market with no way of knowing before hand. I have a ton of Samsung products from household items to gear 360 I was very happy with, but this was a real deal breaker.
I took the watch to service, obviously under warranty and was very surprised the quality of service provided. They took the watch beg it, took my phone number and 3 days later issue was fixed. I see that they had to replace motherboard and battery which tells me something was fundamentally went wrong with the watch.
It looks ok now but I must tell you that when almost brand new product that you really like suddenly becomes useless is quite a trust shaker. I will continue to opt for Samsung however I hope they clear such out of content issues promptly.
Turning off everything suggested in the forum has not helped me either, the battery is discharging at about 4 to 5 percent per hour after the first hour. I did not have a single issue with this watch until just a few ago. I use the sleep monitor and step count only, I went to bed with the watch at 87% battery, and when I woke up (6.5 hours later) was given the warning of low battery. Thought nothing of it, figured I must have hit something to have the display on or something while I was sleeping. Since that point, have done everything in the forum, and the problem persists. There is a problem with one of the last updates, not sure which one, but I have now turned off the auto update feature. This is horrible.
Ridiculous. Just bought the watch yesterday . Configured and was loving it .slept wearing it to see the sleep tracking. Now in the morning I find watch is dead. It was 100 % just 6 hts earlier. Guess have to return this back. Really disappointed