It just cleaned 11mb out of my data about an hour ago - no warning, no reason, but both data counters on my phone (Onavo Count and My Data Manager) are showing thats its whatsapp. I have sent them a report before, this is how they replied:
Thanks for your message.
Sending and receiving messages, pictures, audios and videos with WhatsApp is free, just like sending an email internationally or browsing the web is free. To get the best experience with WhatsApp, we recommend that you have an adequate data plan for your mobile phone or restrict your data usage to Wi-Fi.
Also, please note that you can only use WhatsApp to contact other WhatsApp users (we currently support iPhone, Android, Windows Phone, Nokia and Blackberry devices).
You need to have an internet connection in order to use WhatsApp -- either a data plan or a WiFi connection.
If you have limited data plan, you can monitor your usage by going to WhatsApp > Settings or About > Usage. Lastly, please note that there is no calling feature on WhatsApp; therefore, your mobile carrier rates apply for phone calls. Please also consult with your carrier about using your data plan internationally or while roaming.
When I have checked the usage counter INSIDE whatsapp, its never reported or show the excess data consumption. The numbers in there are in keeping with my typical audio/images downloads. Which makes me suspect it must be auto updating or something - but I cant find any option in the settings for that.