Goats have a hollow place in their side when they haven't eaten in a while. It is in the back third and make a really sunken place. Some people unfamiliar with goats assume it is poor from not eating or has something wrong with it. Compared to a goat that has just eaten, it is a significant difference. But it is a misconception as to the goat's health. One good meal and just hours later the sunken place is gone and the goat looks good again.
A goat is hard to gauge by just looking. The best way to determine the body condition is by feeling with your hands. It takes some practice, but far more effective than a visual check. So just because you see a goat with a hollow spot, don't assume it means much.