This is an example of a so-called "hiatal hernia", meaning the stomach has migrated through the diaphragmatic hiatus into the chest--the stomach is literally in the chest. This example is an unusually large one. Hiatal hernias can cause chest pain, indigestion, and a variety of other gastrointestinal complaints. Heart scans are reasonably useful to screen for this disorder, though very small ones could escape detection by this method.
Sometimes, you can actually hear the gurgling of stomach contents (the common "growling" stomach) by listening to the chest. Large ones like this actually crowd your heart (the gray structure above the circled hernia), irritating it and even causing abnormal rhythm disorders. The dense dark material within the hernia represents lunch.
I would not advocate CT heart scans as a principal method to make a diagnosis, but sometimes it just pops up during a heart scan and we pass it on to the person scanned.