It could be off a site to site vpn or maybe even a remote access one........non routeable IP addy's can be 'routed' in an IPsec tunnel........

It's probably not that but thought I would mention it anyways!

Infact thinking of it....thats the only way a not routable IP can reach it, unless the Outside interface is on a [routed] perimeter subnet and not directly connected to the outside world......

This can be confusing in the logs and running config as 'internal' IP's show up on the outside interface.