Can fluid in the lung come back after it looks gone on x-ray ?

Yes determine cause: It is important to be sure the cause of the pleural fluid is found and treated. Effusions are not always seen well on chest xray and a ct scan or lung ultrasound can detect if there are remaing pockets of fluid (known as loculated fluid). Numerous causes of pleural fluid but important to make sure to evaluate for pus in the fluid, rule out tuberculosis, malignancy, heart failure, pneumonia...