This doesn't really answer the question, as much as bring up another: if you don't know where your WiFi is coming from, how can you have permission to use it?
Back to finding the location: all you can really do is walk around with a WiFi detector (or just your MBP) and track by signal strength (without getting into using actual direction finding equipment). I assume there's nothing unique about the network's name (SSID), but if so, it'll help if it comes to knocking on doors.