My iphone 4S suddenly developed the battery drain problem last week - like it went from 65% to dead within 3 hours, on standby the entire time. A few days earlier, my mom's iPhone also developed the battery drain symptom... Like it literally started happening on both our phones overnight.
I didn't download the latest update until this happened - thinking maybe that was my sign from Apple to finally update my software. Unfortunately, that didn't help the battery problem - though it actually created a couple new ones. (My mom's phone already had the latest update though...)
Out of all the posts I'd read with suggestions and steps to possibly fix the battery drain problem, there was only one that I hadn't already tried... One guy with the same problem made a comment about his work email account, which is set up as a Microsoft Exchange account on his iPhone. It turns out that I also have a Microsoft Exchange account on my iPhone, and so does my mom...
Since I was out of options - as well as battery juice, I threw caution to the wind and deleted that email account... And guess what?! My phone went back to it's normal, lengthy battery life within a minute of deleting it. (Hallelujah!) I can live without those emails, but I can not live without my phone!
My mom thought this problem was a sign that she just needed to get the new iPhone, and that's excatly what she did the following day! :)