The coffee shop may have its own wifi but does it have its own internet connection? If the coffee shop's wifi is connected through the main connection, someone with a coffee shop login could be using it out of cafe hours. Even without a coffee shop login, WPS is far from secure. It is possible to get in without the need to push a button.
However, I think that it could be someone in the office. Or several people. A few windows updates and a bit of watching youtube or even iPlayer would easily eat up that much data.
But that wouldn't account for the use when the office was closed.
Does your router not keep track of data use? Check it on a daily basis and compare with what BT says you used.



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