hI IanNel
Welcome to the forum.
Does your customer visit any streaming websites such as: You Tube ?
When last did your customer logon to the router to view internet usage(Clear internet traffic begin of every month - that way you have a more accurate indication of how much traffic he/she is using/)
Secondly - Under Product Information (View device list - this will indicate all devices that have connected to the router over the months - if you notice an unusual device - then someone unfortunately managed to get the wireless password)
How many devices connect to the router ? Example iPad, Smart Phone, etc. Make sure all devices firmware are up-to-date and disable automatically update apps. (Disable Google+ if your customer is running it)
Does your Customer connect via Ethernet or wireless ? If wireless change the password immediately, don't have an easy password.
When last did your customer do full system scan ?
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I have a customer that is not running torrents or anything and use to have a Adsl (4mb) with a 10 gig cap. now they using 40 -50 gigs in two weeks?