Data theft MTN

Good day, I want to lodge a complaint of my Data been deducted from my account, I use wi-fi 90% of the time but yet my data keep been deducted, last month I loaded 3g and in no time it was finished? I did a test by switching of my data and taking a screenshot the next morning I had lost 1.4g of data, please explain and I have checked all my app they are not using data once off, tell me how I loose 1,4g when data is off? Regards Matthew Erwee