Mysterious Data Usage


My wife owns an HTC Thunderbolt.  We are experiencing mysterious data usage. 

The data log shows data usage during hours and at times when the phone is not being used.  We have tried closing all apps and resetting the phone.

Still, these nickel and dime data used entries are adding up fast and bring us to the brink of our our plan which we have already extended.

We do not stream video, we do not listen to Pandora.... The phone is used strictly for texts, emails, and phone calls.

Any advice?  Thank you for your help.