Wireless Unknown Stations
A wireless station not listed in the Expert Threshold settings for this event has been sensed. Such a rogue station can either be a legitimate user who needs to be added to the list, or an intruding "wardriving" your access points for malicious purposes.
Possible reasons for the event:
The list of known stations needs to be updated to add a new user.
An intruder is trying to hijack an AP for malicious purposes.
Observer is sensing crosstalk between your network and another network within range of your APs.