Interesting question. How to find that rouge wireless connection.

premise:

Access Point is not secure

Even though the AP is unsecured, you know that only 4 mac addresses are allowed to connect to your AP (Don't argue here - I'm making this up as I go)

You notice that 4, authorized, MAC addresses are currently active on your AP; AND you also know one of your four PC's is turned off.

Yea Yea I know how's someone going to know how to monitor an access point and not secure the thing - but hell, it's Friday afternoon.

So how do you find the fourth, unauthorized PC that is connected to your AP using the MAC address of one of your authorized PC's

Also, Assume the offending PC is not in the same building. Think not in your apartment but in the apartment across the street.