Data billing

I purchased 500MB from vodacom @17:55 last eveinig, this morning @ 07:05 I get a message that it is depleted. How can 500MB get finished in less than 24hrs? Firstly I have no proper signal coverage in my apartment and my data service was off for most of last night, my phone was switched off for the whole night with no running apps. There is no way I could have consumed so much data in a short space of time. I called in and was told that it must be an 'update or a app that was open before I switched off my data service or device, I know for a fact that I close each and every app opened before I switch off my data services or my device. I get told by the team leader that whenever you close an app/program and switch of your data services, the minute you switch it back on, the device will continue draining data from your last session even though the app was closed. 2nd issue, I have a 1GB topup contract. My data has always lasted for the entire month often leaving me with at least 200mb data. Recently I have experienced huge data drains even though my usage has been consistent, Can I please have an answer as to why is data being lost. My reference for the SR is S3-9J11D-DQ3L