Are you hooked into any of the SETI stuff?
And could that get into your intranet through your "opening", and use all the computers from time to time?
I have no idea as to how all that works, Robert seems to be on the right track.
A while back my computer was busy sending out information somewhere. My ISP wouldn't tell me where. Ran SpyBot, which I do regularly, and MacAfee's "Stinger", there were no viruses. Shut it down, opened it back up the next morning and it was gone.
Strange things happen, I'm afraid!
Arf