I have had the same issue, and think I may have found a solution. My handset is a Motorola, and within settings / system / privacy there is a section asking whether you are happy to 'share device usage statistics'. This was switched on by motorola (unsuprisingly) but since switching it off the data seepage seems to have stopped (my wifi is switched off at night and I was getting data usage every hour, on the hour, but this is now not happening). So have a look to see whether you have a similar function somewhere on your model? Hope this helps
Thanks for posting in regards to the data usage showing on your itemised bills for individual days/hours.
There are a few reasons this may be happening, most 'smart phones' will access the internet each hour (or as enlli has advised) or during the hour to check for updates for the apps and services you may have on the handset. These can be for emails etc
You can try turning it off in the settings or follow the advice of Drjazz that may help.
Rewind on my previous answer .....that's not the solution. However I think this is :- change the handset to 3G and you should not see any unauthorised data usage. There is a technical reason for this apparently, although not sure I fully understand it, so won't try to explain!