Never had an Iphone but this is something that happens on all smartphones - an application which you are not aware of being ON is consuming battery. These are typically "location aware" applications that use either or both wifi and gps.
Try to find a good "application manager" application and check which applications are ON. Kill the applications that you do not use.
Also, we sometimes tend to install applications that after the first few days/weeks we never or very seldom use - uninstall them.