Can a aortic aneurysm go away without treatment.can a ultra sound miss the aneurysm?

Aneurysm : unfortunately an aneurysm cannot reduce its size. At best, it may stop growing in cases of strict blood pressure control. An ultrasound may miss it if it's in a location that cannot be easily seen. That's why the best modality is a CT scan

AAA: Abdominal aortic aneurysms (AAA) do not go away by themselves. Ultrasound detects AAA but occasionally can miss them, particularly pelvic aneurysms involving branches of the aorta. Likewise AAA can be missed in very large people or those with a lot of abdominal gas as ultrasound does not penetrate gas. For the same reason aneurysms in the chest cannot ordinarily be detected by ultrasound.