I wouldn't take the cat to the humane society, I just wouldn't do that to a animal. It is a cats nature to rome and someday that could be your cat in someone elses yard. (You never know when your cat will get that urge to wander around and check out the neighborhood) I don't think I would even go talk to the neighbor but if I ran into them I would mention it.

I don't let mine outside at all, there is just to many things out there that is to dangerous. To me the soultion would be simple and that is to just keep your kitty inside or only let out with supervised yard time. It only takes ONE time for them to wander into the wrong place and cost them their life.

As far as that fence goes, I have seen cats jump them like the wind. Just because you have a 6 foot fence does not mean that will keep your cat in your yard. I kid you not, I have seen cats jump a 6 foot fence easily!

If you have to let your cat out, build a enclosed safe area for it to be in out there.