Can a hypoechoic area of 4mm shown on breast ultrasound be undetectable 3 weeks later on ultrasound?

Well as no one else has answered my question I will update this situation. The 3 week follow-up ultrasound which the Technician said showed nothing actually shows an poorly defined area of hypoechoicity of now 1.9 x 1.0 cm...upcoming biopsy.