Do you have an access point? Is it secured? What about your neighbor? Odds are, within half a mile of your house, someone has an open access point. All it takes is someone parked nearby a few minutes to connect, upload something, then drive off. Unless you have video footage of your street 24/7, there isn't much in the way of being able ot track them down.

Most virus writers get caught not because their point of origin is tracked down, but usually because of a tip from a friend, or in some cases, because the creator can't keep their mouth shut. I think it was the sasser creator (but possibly someone else) that was caught because of their conversations on IRC and someone sending in a tip.