My iPhone 6s

Probably apps running in the background are causing this, sometimes Mail get's stuck connecting to the mail server, or some social media app like Facebook can also be a reason for prolonged background activity.

Try to force restart the phone by holding the sleep and home button for about 10sec, until the Apple logo comes back again. You will not lose any data by doing that, but it can cure some glitches.

If this does not help, setting it up as new device would be the next step:

How to erase your iOS device and then set it up as a new device or restore it from backups

Maybe also take a look at this page about battery drainage

I have the same problem as I bought my 6s ten days ago. Funny thing is the issue all over the internet but when I contacted apple support, they didn't mention it.

Check this out:

http://appleinsider.com/articles/16/01/16/apple-promises-fix-for-iphone-6s-6s-pl us-not-displaying-accurate-battery-data