Baffling problem even the mechanic can't solve.

Hi, I am new to this forum.  I have a problem with my car. My 1998 Polo is literally drinking water.  The radiator is fine.  The pipes etc are not leaking.  The head gasket apparently is ok or the UV dye should have shown up in the exhaust fumes.
 
But yet the car seems to be shifting around 400 ml of fluid a week.  We have no idea where it is going.  There are no water stains on the floor etc.
 
Any body any ideas?
 
Thanks