your computers
or somebody elses
you should be able to log into the settings/service page of the MiFi
and look at the router log and see data use
you have two possibilities a glitch in verizons billing
OR someone accessed you mifi using a directional antenna
while you are checking the mifi router info
change your password
the girl at the store can't help you
you need to call customer service and ask to speak to tech support
ask for a manager, tell them you need to resolve this issue
GET you mifi data info from the mifi BEFORE you call them
there is the possibility of a malfunction
since this is for home..turn it off when you leave home
heck pull the battery if you want to be sure its DEAD
when your not home