i havn't done any thing to my phone. its stock and i have the same issue.
mobile network randomly turns itself on and uses data. i almost went over this month. this is really starting to be an issue for me.
Not sure what to check, but you probably have some apps that are actively updating...I think there are settings to use WiFi or data for those updates. WiFi turning on should actually save data. I believe you want the mobile network on all the time. But check settings to be sure you're using the networks right.
Kurt