You say you haven't used your computer until 3:30pm - okay. But what about your computer itself - was it left switched on or off?
I put it to sleep when I'm not using it; it was still asleep when I came to use it at 3:30pm
If on, maybe there was some background happening while you was not in front of your computer? (i.e. BBC iPlayer's P2P)
If that were the case I would have had to have left it on from the previous evening (I hadn't) and there would be a massive amount of usage showing in the free period from midnight to 8am, but that is only showing 19.13KB.
The more I think about this the odder it is. Everyday the first thing I do when I get up is wake my iMac check my mail and go on the net. Yesterday I made a conscious decision not to do so as I wanted to get to town early and know from experience that this is impossible once I sit down at my screen.
I do find it a very odd coincidence that a hacker would choose to access my wifi on the only day I am not using it, particularly when one of my neighbours has a fast unprotected connection. Why would someone go to the trouble of hacking into my password protected wifi when there is another instantly accessible?