My new iphone 5 is doing the same thing. I have spent hours on the phone with Verizon (they are telling me it's not happening), but it's showing up under my usage as "Tether Data". It even goes up when I'm just talking on the phone.
I also have contacted Apple regarding this, and they told me they have not heard of this happening. I took it for a genius appointment, they worked on it for a long time (resetting, etc.), but could not figure out why it was doing that.
They proceeded to give me a new iphone to hopefully fix the problem. Same thing is happening on my new one.
Contacted Verizon again saying it must be a problem on their end, but they say it's not and are not helpful at all.
Not happy- have just been keeping cellular network off because the tethering is eating all my data even though I don't even have a hotspot set up!
Any advice would be greatly appreciated....