Been getting the same issue for the last month or so.
Charge the phone up to 100%, check the charge level 10-15mins later and it shows a really rapid drain, sometimes it will sound a warning about 'rapid battery drain'. Thing is, if I re-start it, it shows 100% charge again and everything is fine for a day or so. I've used 2 different battery monitoring apps, and both show the same 'phantom discharge'.
I actually think the battery life is just fine (once you do all the battery saving tricks..... ) and this is some annoying software glitch.
This is my first smartphone of any type so I have no point of comparison, but aesthetics and camera aside, little bugs and annoyances like this are making me feel I should have purchased an Android or IOS device. Well, that and the Ipad I bought around the same time has been far more consistent and bug free in it's operation.