I don't know the complete answer, but mine does the same thing about half the time. Sometimes it doesn't loose any battery charge and sometimes it loses about the same as yours. I understand why part of the times. It is because when I shut it down the icon LEDs are still lit. They stay lit unless I start the phone again and turn it off. However, I can't explain why it happens other times. I charged my phone 2 days ago and never turned it on yesterday. When I turned it on this morning the battery was at 53% and the LEDs weren't on.