power steer fluid loss

I have a 97 B5 Passat.The power steer fluid has lost half a liter twice with no trace of where it has gone!This is enough to make the pump groan & take it to the shop.It has 105000km (65 miles) & recently had service/ cambelt/ tensioner/ water pump etc.
Mechanics can find no fault!!

v6 or 1.8t


Fluid just doesnt "dissapear" it either leaks, or leaks..