I noticed that mine went from 93% to 90% charge while asleep (both of us) for 6 hours. In settings/ battery, tapping on the graph, there was a red line that disappeared when I disabled nfc. You can also see small spikes of activity during screen off. Maybe different sleep levels, sleep, deep sleep, rem sleep, comatose, yawn.
Turning the tablet off seems the best solution for now to hold a charge.