Today was the first time I faced the battery drainage issue and I think I know why, at least for my case.
My college campus has an unprotected Wifi (No WEP, WPA etc) but requires me login in with my student ID when I first try to visit a site.
Today I didn't login in but the phone connected to the Wifi. It went from 70% to 50% in less than an hour, no usage. I logged in 3 hours ago, and currently at home. Battery's at 44%, a 2 minute call in between.
So I'm guessing that since I didn't login when on campus, the phone was continuously trying to do something through wifi but couldn't?