The same thing happened to me. Series 4. A month or so after I got 5.2, my battery started draining super quickly. After normal use, I used to get 60% or so leftover battery at the end of the day. After the drain started, it drained and turned off on its own by 7-8pm.
I took it to the store, they of course ran 2 minute diagnostic and they said the physical hardware is fine. They gave me the standard battery saving "tips", even though my usage patterns did not change before and after the issue started.
I tried a complete wipe and reset as new, didn't help. I wiped again and did not install any third party apps, I turned off background refresh, I kept it in airplane mode, I did not set up cellular, nothing helped. I upgraded to 5.2.1 when it came out, same deal.
I took it to the store again, they ran the same diagnostic, the person assisting me said "I see something in the background is draining your battery". The person assisting me gave me the same stale tips about apps and background refresh. I insisted that those did not help me, and then they shipped it off to the repair center. Less than 24 hours after they got it, they sent it back to the store with "couldn't replicate issue". I assume they ran the same superficial diagnostic instead of wearing it for a couple of hours to see the battery drop dead in front of their eyes.
I'm about to go pick it up from the store today, and I am not happy at all with how it was handled.