Dissapearing data

I have a 10GB monthly contract (residential) and over the last 2 months my data has been running out by the 20th if the month. I therefore have closely monitored it lately and I know that the data usage apps is probably not 100% accurate but when my data ran out this month I added all the data used by the devices that are linked to the router and I got to a usage of just over 5GB. Where o where do you think my other 5GB has disappeared to. <br> <br> My router could also not be hacked as I live on a small holding and the closest neighbour is more than 250m away. If this continues Vodacom I need to relook my contracts. Maybe I should start a blog where every customer can log their usage and disappearing data.