Could an ultrasound miss detecting abdominal aortic aneurysm?

Yes: If the patient is large and/ or has a lot of gas in the bowel then the aorta might not be well seen. Such an exam would be described as sub-optimal and could not rule out an aneurysm. If the aorta was well seen from above the celiac artery to the aortic bifurcation then no significant aortic aneurysm should be missed.

Dx of AAA: Ultrasound is one of the typical studies used to diagnose aaa. It is optimal to let the tech doing the study know if you are trying to look specifically for an aneurysm. The bigger the aneurysm, the more likely it is to be picked up. In other words a very small one would have a higher likelihood to be missed.