I've got a pretty serious bug on 6S Plus...
I noticed my phone gets warm sometimes, with no use. The only app I have enabled background sync is Whatsapp. I don't even have Face Time or iMessage enabled. iCloud photos is disabled, as well as iCloud Drive. Find my iPhone is enabled and that's all. Not even any kind of push is enabled. Though I found that Documents & Sync under mobile data usage in System Services is using tons of data whilst on standby. This is consuming my mobile data plan and I don't know what is causing that, but it only happens on 9.2 (beta 2 and 3).
First I thought that could be the backup restoration I did from my 6 Plus, so I erased my 6S Plus and restarted clean. Still, the problem persists after some hours. Actually I tested this on 2 different 6S Plus and I have the same problem. WiFi assist is off.
I also thought it could be Siri, but "Hey Siri" is off.
I'm going back to 9.1 and I'll stay there until I'm sure this is fixed.
If you find anything unusual with your 6S on 9.2, keep an eye on mobile data usage on System Services/Documents & Sync. If you're using WiFi, you won't notice. Please, report this bug to Apple as it can quickly become critical for many users.