1999 Other Suzuki Models Water level in Reserve Tank

I bought vehicle used 3-10-07. It's a 4 cyl "Vitara" 2/4 wheel drive. So far, great running vehicle, except for 1 thing: the water in the reserve tank keeps disappearing. There are no visible leaks at all! When I first got vehicle it took 6 months for the bottle to go empty! I refilled. Then it took 6 weeks. Recently, it seems to go maybe 3 weeks! Why? My engine never overheats, unless tank is dry and I am idiling. I have no gas mileage loss and still have excelleration power. I thought it could be the summer heat/evaporation, but it does it even in the winter. I can't figure it out!The vehicle runs great except for the mysterious loss of water in the reserve tank?