I've had the same concern for a while now. Short of actually breaking the law by connecting to their network and placing a message on their PC, I think the only way of locating it is to walk the neighbourhood with your laptop - at the risk of looking like a really blatant wardriver! - and regularly refresh your network list to see where it's at its strongest. You might have just a few candidates then.
The link points to a program called NetStumbler which is used to analyse wireless networks.
I use it a lot at home because there are a few wireless networks in the area and I use it to see which channels they are transmitting on. That way, I can make sure that I am on a different channel to minimise interference. There is an option on NetStumbler to identify the directions and distance of a wireless network if you have a GPS antenna connected.
Alternatively, drop a note through several doors in the area explaining the problem and offering your assistance (you might make some new friends).
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