Unknown IP Address

You can probably jump into the CLI and run a #get system arp

That may show you the MAC address of the device. It may be a device that has two IPs which you can then track down. Also you can try turning on device discovery on the interface where the IP originates from. This will only work if the firewall is the Layer 3 gateway for that network. You can do this from the CLI, only change the port1 to your interface:

config system interface

edit port1

set device-identification enable

end

You can also enable that in the GUI using Network -> Interfaces -> port1 

Now check the box for device identifcation and hit apply

You should be able to browse to user & Device -> Device Inventory (in version 5.4) to view what the firewall has found. 

I hope that helps.