What's eating up my iPhone data usage?

I have an iPhone 4s on the Orange/EE network, I have a 500mb allowance but orange have been charging me for an extra £10 every month and claiming it's data, here's the funny thing - I always turn off mobile data and I only use Wi-Fi on my phone, could it be location services? I've turned off other things in case it could be that.

Added (1). EDIT: I always close my apps.

Double click home button and close out of your apps

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You need to exit out of every app that pops up on the bottom when you double click the home button and put your phone on airplane mode when your not using it which you can find in settings

Hi 500mb could be used when youre outside?
Do you do that often?
If not it is EE's wrong and you should call them

Do you have background data on? That might be a problem. Also turn location services off.