Just because you connect with a cable does not mean your wifi hub is not wifi ing its little transistors off.
Read all about BTFON here
BT | FON
This might be some more useful information for you
How to disable the stupid BT FON..? - BTCare Community Forums
Even when not is use wifi access points still transmit to help users find them.
Then again you could have people all around you using wifi or wireless video extenders, bluetooth on their mobiles and the 100's of other things that transmit radio signals.