Hmm, that is interesting. If you've used up 9 GB, your phone would have registered which app would have used it (unless if it's software update as Zoe mentioned).
In saying that, Android phones have a Data Usage functionality, where you can set a warning once you've reached a certain amount of data usage. You can also set the phone to automatically turn mobile data off once you've reached another amount of data usage (set this just under your data limit for the month). Not a total fix, but a nice safeguard.