Finally found this in the docs. This occurs when the AP fails to contact a preset IP using ARP requests. Have changed this to a custom IP on my network so I at least know where these requests are...

I have 4 EAP225 AP's controlled by an OC200. This morning in the logs on the OC200 was an entry saying "AP - 4 has been isolated" When I looked at the devices page, the AP in question had its status...