The power button just puts it to sleep so it's not really powered off. you could run powercfg /sleepstudy from an admin command prompt to see what was using power. I think. cd to temp folder first so the report is easy to find.I just charged the M3 to 100%, turned it off using the power button, and turned it on again after about 40 minutes and it's at 95%.
Should I return it for a new one or what?