i had this same problem suddenly appear on my allroad. i pulled the fender liner and looked around and found that the line from the tank had separated from a splitter that directed the fluid to other lines. the small plastic clip that 'held' the line on had come loose and fluid was leaking ever so slowly. i refit the line and the clip and all is well.
and by that i mean now i notice that the drivers side headlight washer line also seems to have come loose and i will need to get under the car again to investigate that.
but! the tank doesn't drain every 2 days anymore, i just avoid using the headlight washers