If you aren't streaming Netflix/hulu/Amazon VOD, then the only other possible data hog might be a service update download. If your Tivo is very new, then it would have done a service update probably within a week after being hooked up. Not sure how much data that is, but couple that with the initial Guide Data download.... If it is a Premiere and you are using the HD menu interface I think that pulls internet data as well in real time.
"I'm just one story in a two story town." Bon Jovi