Cyst: A simple cyst is a common finding in various organs and can appear as a nonvascular hypoechoic structure on ultrasound. The size (0.4x0.6x0.9 cm) is consistent with a small cyst, which can be benign and often found incidentally.

Abscess: Although less common than a cyst, an abscess can also present as a hypoechoic structure. The absence of vascularity might suggest a contained infection or a early stage abscess.
Hematoma: A small hematoma could appear hypoechoic, especially if it is not highly vascularized. The size and shape could be consistent with a resolving hematoma.
Lymph node: Enlarged lymph nodes can be hypoechoic and may not show significant vascularity, especially if they are not highly reactive or malignant.

Malignant tumor: Although less likely given the small size, certain malignant tumors (e.g., metastases, lymphoma) can present as nonvascular hypoechoic structures. Missing a diagnosis of malignancy could have significant consequences.
Infarct: In certain organs like the kidney or spleen, an infarct can appear as a nonvascular hypoechoic area. Identifying an infarct is crucial due to its implications for organ function and potential need for intervention.

Parasitic cyst (e.g., hydatid cyst): In endemic areas, parasitic infections can cause cystic structures that appear hypoechoic on ultrasound. These are less common but should be considered in the appropriate clinical context.
Congenital anomaly: Certain congenital anomalies, such as a congenital cyst or a remnant of a embryological structure, could appear as a nonvascular hypoechoic structure. These are rare and often depend on the specific organ or location involved.