Car is losing power steering fluid very slowly. Months go by before it needs to be topped of again. No apparent leak, no noise, no foam in fluid.
Check inside the boots on your steering rack. With your vehicle properly supported on jack stands, remove the wheels and take a pair of pliers and squeeze the clamps on the outer collar of the inner tie rod boots on the rack and slide a small pocket screwdriver or something similar CAREFULLY between the tie rod and the boot collar to see if there is any fluid residue inside the boot. There should be none at all. If there is any amount of fluid the outer seals are leaking and should be replaced. It should be dry or very close to it inside the boots. The seals can be replaced without changing the rack (I have done it myself) but you will be hard pressed to find a mechanic that will do that because it's faster to just change the rack.