L.T.
So you have a 17 year old dog? Wow! What kind of dog is it? When I was a little girl we had a basset hound who never left the yard, we did not have a fence but he never went out of the yard. Anyway, suddenly when he was very old, he would leave the yard and my mom would have to go find him. The last time she went to find him, he had collapsed and died. One thing that really bothered me then was that he died alone when he had a family who loved him. I have heard that dogs will do this when they know the end is near but there may also be some dementia with your dog. Maybe she just doesn't know anymore that she is not supposed to run off. Poor old girl.