My VW Passat is leaking water.?

I have taken it to a mechanic who is baffeled & finding it hard to trace where it's coming from. It is occuring front left hand side under where passanger's feet would be, but could be running down the bulkhead & ending up there. The water leak is not coming from the coolant or washer bottle. It is not oily and is tasteless to the tounge. I thought it could be the heater matrix but there is no water in the passanger footwell either. Does anyone have an idea what it could be?????