Ceri27b

Hi after the car standing for 10 days ,in June! I used it & it slightly overheated. When cooled I checked & coolant/water tank empty I re filled no overheating & no problems. However 2 weeks later I checked again & tank was again empty, refilled again checked couple of days later again empty?! There has been no more overheating, fan comes on , no visible water marks under car, around engine etc. so can somebody please tell me where my water is going. No blue smoke from exhaust, oil is fine! 2004 lexus is200