Power Steering Fluid Seems To Be Leaking

[omeo]

I have a lexus is200 se. My issue is with my power steering fluid. In the power steering pump I have fluid in there however this is quiet below the minimum cold line. When I top up my power steering fluid to the desired level (between the min and Max cold line), turned the engine on And checked the fluid about 5 seconds later the additional fluid i added was gone from the reservoir . The strange thing is that I only loose the additional fluid that I put in, the fluid that was already there before I topped up remains at the same level always.There is never any change to the amount of fluid that was in there before.

Does any one have any idea whats happening? I would expect to loose all the fluid but i only loose the fluid I top up ?