Unknown IP address shown in Log Report

Hi Forti Pro

I need your help. the attached files are the screen shot taken today from fortigate 100D  log and Report.

I've never seen that before but this morning when i enable the captive portal for wireless user associated with Radius server configured to authenticate user, after some test the unknow ip address appear to my log report it alerts me a bit. Now i am wondering if this a threat or not?.

Thanks for the help and much appreciated.

Regards

ian