WAN IPs occasionally show up.
It could be anything.
Maybe just a heartbeat from your provider checking you're still on.
https://lantisproject.com/downloads/gargoylebuilds for the latest releases
Please be respectful when posting. I do this in my free time on a volunteer basis.
I'm a little surprised that this didn't concern you more. I've never seen external IPs in the connected hosts list before, with any firmware, ever. Maybe other FW just hides this information? In which case good for gargoyle for including it. If it is just gargoyle, there's something odd going on that should probably be dealt with.
WRT1900acsv2 / alas, back to DD-WRT
WRT54G /DD-WRT v24-sp2 (08/12/10) mini