Tethering data leak over Powerline iPhone

I have an iPhone 3GS with AT&T 2GB data with tethering. I usually leave USB tethering on (Bluetooth off) continuously because I frequently tether my laptop.
Lately I have noticed large blocks of data usage (9-12 MB on average) occurring overnight while my phone is charging via AC adapter (NOT laptop USB). My phone connects to Wifi and rarely uses 3G data at home. But I do have a Powerline network throughout the house connected to my router.
Is it possible that the iPhone is somehow tethering to a computer in my house via the Powerline network through my AC adapter? I cant think of any other explanation for the data leak. Last night I made sure tethering was turned completely off, and there was no significant data use overnight. Very odd.
3GS iOS 4