My 928 still suffers from intermittent, device-warming phantom battery drain, hoping 8.1 fixes it. Luckily the "Battery Saver" feature seems to be a so-so stop-gap if you don't mind missing email notifications.
I had this issue a few times, and each time it was due to an app being stuck on an initial setup screen. Going through the prompts solved it. Strangely I had the same issue on android a few times. My 920 gets two days of battery life.
