Over night with out using phone anything 400+ MB

hi guys this is my second complaint in successive months regarding data missing or *****ing from my account with out using, this time over night phone didnt updated anything not used and my updates only selected to wifi even though i woke and noticed in the morning more than 400mb used before i got up from bed, that is very strange and surprised to me, i am not sure how to react the last time also they saying iam using but i didnt used so eventually they added me complimentary 500mb but now this seems to be serious issue how is this happening with out using interenet data is missing, it is a serious threat to the customers. i need an explanation ASAP.