Our Verizon Mi-Fi just did the same thing. It was using gigs even when we weren't using the puter. We have the 8G a month package and haven't gone over that in the 3 years we have had the unit. One day it used 13G. How in the heck can it do that?
I use puter for email & surfing the net. Very rarely look at 'you tube' and the like. Wife called Verizon and they had no explanation but sent me a new unit (arrived today) and 2 days before our billing cycle ends, we are to call them and they will make a billing adjustment.
It used 1G while my wife was on the phone with them and all I was doing was playing an installed game (no internet needed).
DICK