Well, have you tried again to find the owners or just at the time that you found the ring? We didn't have stuff like Craigslist back then. If you haven't tried to find the owner in the last year or two, I'd give it one more try. After a few weeks if no one claims it, I think it would be fine to...
I know she has a big yard, but is she getting enough exercise? Labs are very high energy dogs and sometimes dogs that have a lot of unused energy will start doing strange things when they get bored.