I have an HTC One (M8) with Cyanogenmod 12 (12-20150625-snapshot-yng4nao09n-m8) which I have been using since June. Before that I had a different version of CM on the same phone and did a full reset. Late September, high data usage by the Android OS system showed up like the bogeyman out of nowhere. I've had a 3GB plan for 2 years and have never once gone over until now where I am over by 4GB.
Verified Backup and Sync is turned off for the phone and for photos. I also cleared the cache and data for Google Backup Transport, Google Play Services, Google Play Store, and Google Services Framework, rebooted, and thought I had the problem solved, but installed My Data Manager and waited.
The built in Data manager shows everything as Android OS, with no option for me to restrict. Data Manager caught it happening today and says its EasyAccessServices. What is EasyAccessServices, anyway? In All Apps the option to disable EasyAccessServices is greyed out.
Every data jump has been in the same amount: 0.64GB (650-680MB). It will happen at different times of the day, on random days. This has got me tearing my hair out in frustration. I've got some rollover data built up but I can't afford too much more of this.
EDIT: Its been explained that EasyAccessServices is the pointer/gesture app built in to the OS that basically acts as the touchscreen driver. I can't tell if the data was uploaded or downloaded, but maybe the app is trying to update itself and failing. Looks like
com.htc.easyaccessservice is version 2.0.831094.
EDIT2: I can Force Stop the application. When I do I still can use all the gestures I can think of in NovaLauncher and Facebook.
(Click image to enlarge)