I have an Aunt and Uncle that stumbled upon a similar situation with a home they purchased. They lived in the house for a couple years before deciding to install a fence around the back yard. While having a roughly 40' deep section of wooded areas cleared out, my Uncle began to clean up what he thought was just a pile of old wood, bricks and stones. Come to find out, someone intentionally put a pile of trash over a door that lead down into a shelter.
Based on the pictures they took, it was a bit larger than the shelter in this news story, but it was set up very much the same way. After inspecting the contents, they could tell it was built in the late 50's or early 60's. There were four beds, and they estimated enough food and water storage to last four people at least a month.
With the help of some of the contents, they were able to find very little information about who owned the home at the time. They did determine the owner had something to do with Fort Stewart, but not exactly sure what.