This morning I purchased 250MB of data from AT&T ahead of the auto-renewal schedule because I had used it up two weeks back. I then checked a few emails without downloading attachments and rode to work. Didn't use the iPad at work, though it would have WiFi there anyway.
About 2 hours after the purchase I receive the usual "only 20% of data left" email and am astounded by that, but go to lunch anyway. When I return from work I have the 10%, 2% and no data left email in my inbox.
I have iCloud Documents (though I only have a few of them), FaceTime and Reading List activated over cellular, but not iTunes. The iPad had been synced the night before, I was not signed in to Skype or similar and I actually didn't use it.
Anybody an idea what might have caused this??
I talked to the AT&T help line and all he was able to see is that the data was consumed in 5-6 bursts, not tickling over the three hours. But which service, no idea. I'll visit an Apple Store and check with them, maybe they have means to determine what went awry.
Best Answer
If you use Photostream or iTunesMatch there is a good chance that one of the two uses cellular data unexpectedly. As far as I remember there was a problem (before iOS 6) with syncing these two services: when you started the sync in WiFi mode and then switched to cellular (e.g. because of leaving from home), the sync would continue independent of the setting disallowing usage of cellular.