1999 Taurus Power Steering loosing fluid

My 99 is loosing Power Steering fluid. There is not one drop on the garage flloor and no obvious residue on any steering component. The pump is actually making less noise now than before it was loosing fluid. The only time I hear anything is when the fluid is low. Has anyone had problems like this? If you were to try some things, what would they be? Thanks in advance.