great pyr. who has started roaming far from home

We have a 1 yr. old Great Pyrenese male who recently started roaming while we were away for 2 weeks on a trip. We had a housesitter staying at our place. The housesitter took the dog on long walks, away from our property, and ever since we returned it has wanted to go on walkabouts without us.

We live on 14 acres bordering national forest and cattle grazing land, and the dog has always had free rein, never confined, because we never had a problem before now. We have other animals horses/mules/pigs and another dog who he seems content with. We were getting calls from rancher neighbors who threatened to shoot our dog if they saw him on their property, so for now we have chained him, until we figure something else out.. Since it seems nearly impossible to completely fence our property and we really hate chaining him up, though he never barks or seems to mind. From our perspective he has plenty of love and stimulation (I'm always at home and he has bonded to me) , he sleeps at the foot of our bed at night, seems happy and well-adjusted, has learned basic obedience, loves and protects all our other animals. So, we're not sure why he is wandering..... he is neutered and loves playing with our 11 yr. old german shorthair, but now they wander together. They like going out into the forest to find dead animal carcasses (deer, elk) and sometimes bring a bone home. That wasn't so bad, but today they roamed 10 miles from home, traveling along highways where they have no experience with cars and traffic. Running around in the nearby woods is one thing, but now they're going out on roads and in fields where they are calving, so we must put a stop to it.

So, if they do roam and we get a call and go pick them up, how should we treat the dogs? When this happened today, we picked up the dogs about 8 miles from home, drove them to w/in 2 miles and made them run the rest of they way (they were already very tired from their long day of running) and then made them sleep inside the laundry room, instead of in our bedroom. They also didn't get any dinner. But they were so tired, I don't think they would've eaten anyway. So now what? Any ideas for us?