Try Innsider (Score 1)

Try Innsider if Netstumbler fails. It is free and friendlier with WIFI cards. At least you can see if it is a WIFI signal. If it is interference and not a clear WIFI signal, then invest in a cheap spectrum analyzer like WI-Spy. Anyone have an ultrasonic alarm system around? http://www.metageek.net/docs/inssider-user-guide