I suppose even if in airplane mode, you could still have a high % of battery drain if you got apps running in background, glance on, etc. I guess some apps might still be trying to communicate/download some contents from the internet and it is not able to (as you in airplane mode), but it could still repeatedly try again and again if its not well written.
To be honest, the best thing is to hard reset, and don't install anything, turn off everything, don't add any email accounts or any other social media update, and like me, you can lovely achieve "only" a lost of 11% overnight (8 hrs)
my lumia 520 can last about 8 days!