iPad Air 2 used ~2GB of Mobile Data Overnight: what for? I just checked my email and have had a shock. Two emails received from O2, one at 6:39 am warning me that 80% of my allowance has been used, the other warning me at 6:45 am that I had run out of data allowance completely. I had almost my full 2GB allowance left, since it just renewed on Tuesday, and so that means something downloaded ~2GB of data on the 3G/4G connection!

I can't figure out why. 6:39 am was around the time I left the house this morning, so it would have been using 3G/4G, but the iPad was in my backpack and I was therefore not using it; so why/what was it downloading? When looking at the mobile data usage (I reset the statistics on Tuesday) I can see that 2GB has been used this period, but there is no breakdown about which apps have used the data.

One other thing to note is that I had set Sleep Cycle to wake me up, but when I did wake up the alarm was no longer set and unlocking took me to the home screen. As if the iPad had rebooted overnight.

I want to find out what's happened here, but I am unsure how to find out any more. If iOS' Mobile Data statistics won't tell me which app did this how can I find out?
2Gb is in the range of a full software update (or a full backup), and it's unlikely the iPad did that over cellular.

Did you have anything tethered to the iPad? A laptop perhaps? The data limiting doesn't kick in over tethering, and the tethered device thinks it's on wifi. 

If that is the case, you may want to turn off auto-update.