I know that the iPad will sync wirelessly with your PC if you have iTunes open. I had my iTunes set to open automatically, but after my iPad opened my iTunes 3-4 times in an evening, i turned that setting off. So, despite it supposed to be that the iPad would wirelessly sync only if plugged into power, that's not the case. So if you have wireless sync active that could be part of the mystery drain.
If you have iCloud active, it may be doing the same thing. Actively syncing while you think it's just sitting there.
Nothing but pure speculation but there have been some that have said they experience better battery life with iCloud syncing turned off. Could be something you can experiment with.