Apps eat data in the foreground and background just sitting there. So if you are hooked up to your phone that's were it's going.
Just bought a new phone with unlimited data BUT with a cap where they throttle you back.
That said. I don't search the web etc and in 5 days I was at my max of data with out me even using it!!
Went into all the different settings they hide within and found all the apps the phone came with were using MEGA data in the foreground and background. I'm still finding them.
My phone allows me to disable the usage in the foreground and background. They bear a warning that if you disable them other things won't work. That simply is not true.
I disabled almost all of them and the data usage stopped adding up. I also keep the data shut off at all times.