We learned of this one day when I saw Starbuck running past one of our windows. Thankfully, River is too timid to try to compress her entire body under the fence as she is more of a flight risk than Starbuck is. Starbuck came back when we called, wagging her tail happily.
I discovered that she had dug out an area under the back gate to the dog yard. To discourage she from doing this again, I filled the hole with heavy rocks. This seemed to hold her back until just a couple of days ago when we again saw her running happily past the window. Somehow, she had managed to move enough of the rocks to still squeeze her way out.
Thus, I dug out that area a little more and dropped a portion of one of our fallen trees into the area and then covered it back over. They seemed disappointed when they checked out the area later. I'm hoping that will be the end of their escapes.