A device may appear on the Event Log but not be visible in the clients list. Searching the clients list will also not return this entry. This occurs when a device attempts to connect to an SSID but is not able to fully connect. If you click on the device in the Event Log there is a page for that device but the traffic graph and pie charts will be blank. As illustrated below the access point never fully saw that device as one of its clients, thus it does not populate the list.