What was found
IMMEDIATELY AFTER TAKE OFF , PASSENGERS STATED SMELLING AND SEEING SMOKE FROM BEHIND THE PILOT SEAT. FLIGHT IMMEDIATELY RETURNED TO BASE. MAINTENANCE DISCOVERED A HYDRAULIC LEAK UNDER THE ELECTRICAL JUNCTION BOX. TROUBLESHOT AND INSPECTED LOCATED A HYDRAULIC LEAK IN THE GEAR DOWN LINE UNDER CENTER FLIGHT DECK FLOOR CAUSING MIST AND VAPOR SEEN AS SMOKE. REMOVED AND REPLACED THE HYD LINE. OPS AND LEAK CHECKED GOOD.