Had this issue months ago - personally, I feel Comcast is creating this problem so they can move you (charge) to their "unlimited data" plan. Which I finally succumbed to. Had outrageous data useage for a few months for no apparent reason - of course, if you pay for their X1 modem, they promise to be able to teack (I was using my own approved purchased modem). I purchased google wifi mesh router network in my home ot be able to track useage. One thing I would check - if you are using a device from your cell company (I had a Tmobile "Cell Spot") - apparently those devices act as a virtual cell tower for your cell phoine company and you cant restrict who can acceess data cell network through it - in other words, your neighbores could be watching movies using their own cell (non -wifi) and it may be going through your Wifi without your (or your neighbors knowedge) and using your data. The cell companies give these out free ususally to avoid having to build out their network where customers are experincing challenges. You cannot restrict who uses the "Cell Spot" its essentially a mini cell tower for anyone within range but goes over your internet data connection. Onme thing to watch out for. Ironically, asd soon as I subsribed to unlimited data, I've never had another month where data useage was high like before - how convenient Comcast.
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