I have a 2015 A1502 13" Retina MBP, originally running Sierra. Since upgrading it to High Sierra, it doesn't seem to sleep properly. Before, I could leave it in sleep mode on its battery for over a week and still come back to a decent amount of charge. Now it seems to only turn the display off - after leaving it unplugged for 24 hours (wifi off), I opened it today to find the battery at 25%. Power Nap is disabled. I do have Caffeine installed but that's also switched off.
I tried pinging it after closing the lid and continue to get a response from the Thunderbolt ethernet adapter long after it should be inactive, even with Wake For Network Access switched off. Ping follows a pattern, about 25 successful with sub-millisecond responses, then drops several, then the next successful packet takes >2ms, then back to normal <1ms for 2-3 packets, then back up to 2ms. Both computers are cabled at this point. On the other hand, it doesn't respond to SSH while in Sleep mode (as expected).
I don't think it's anything to do with the ethernet adapter as I also see the battery drain when the computer is completely unplugged. Obviously with no indicator lights or moving parts on the computer I cannot tell whether it's active or not, just relying on the screen to tell me.
I have a 2014 15" Retina at home and the battery on this is astonishing, managing 2 weeks in sleep mode and still usable when I come back to it. It's running Sierra. I'm very hesitant to upgrade it to HS in case I lose this ability. Is this a known issue with HS?