Retrocardiac lucency with air-fluid level is seen in
The gastric fundus when displaced above the diaphragm, presents as a retrocardiac mass on a chest radiograph. The presence of an air-fluid level in the mass suggests the diagnosis. ref - Harrisons internal medicine 20e pg 2211
Hiatus hernia
Distal end esophageal obstruction
Eventration of diaphragm
Zenker's diveiculum
G.I.T