Well, dependiong on the brand of AP it might give you the MAC address to look for, but unless you can put it together with a user that's having a problem getting on the network there's no way to find out where this MAC came from. It's up in the air, so to speak.

Does your Ap tell you what the MAC address is? If so, does the UID match on e of your devices? (A UID is the first part of the MAC address identifying the manufacturer of the NIC.) IF it is then you should get a call from someone who can't connect via their wireless card, if it isn't then you might have a n unauthorized user trying to attach to get free high speed.

Hope that helps.