Is it possible to shut the hub down completely on a Friday afternoon and restart on the Monday morning? Ask BT for usage figures at both points in time and they should be identical. If they are then it points to an unknown user, if they are different then BT will need to explain.
Can you instigate a turn off at night regime too? Draw up an process for the last to leave to turn it off and first to arrive to turn it on ... and initial a list to confirm. Then start to review daily usage stats.
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