This issue has been discussed at length in this forum. So you don't have to read a bunch of posts, here are some tips. ATT will post data usage a few times a day, so this could be several hours usage posted in one bunch. IMHO this dosen't add up to such a large usage figure. WiFi will stay on in sleep mode if your iPhone is connected to a power source, so plug your iPhone into a power source at night. You can also turn off sending Diagnostics & Usage Data to Apple. Settings > General > Diagnostics & Usage. You can also turn off push notifications. Turn off cellular data at night. I know that you may not want to use all the tips. I personally plug my iPhone into a power source at night and turned off sending data to Apple and this has kept my cellular data usage very very low.
I had the same issues. I turned off all the setting that people suggested, but I still see high data usage. Strangely I observed that even when I am keeping my ipad on airplane mode for a complete a day with all applications closed also, I have seen the high data usage in the morning my time between 8-9 am(typically). The only way I could stop was turning off the iPad, which I do not want to do that.
I have 5gb plan, More than 50% of my data usage goes for these kinds of untracked Apps and leaves me little data for use. I thought apple is a safe device, apparantly it is not. Since I am seeing this happening when all the apps closed and airplane mode, I also suspect if apple gathering any data from the device.
Not sure how to get rid off this