In terms of spotting the culprit software, then its the actual live data from recording a TCP log, or the Sites Visited page that will be of most use.
YouTube may be eating more data if the higher quality clips are being picked e.g.
Shows difference between old LQ youtube, standard and HD. Also sometimes if using flash for streaming it has been known for you to leave a page, but the data to be still streaming. Have seen this on both a PC and a Mac
Add to this the software updates that are becoming more and more numerous I can imagine a home with several PC's using a couple of GB a month on that.
Others have had issues with the BT system recording usage even when everything was turned off, so their system is not 100% foolproof