I have recently had wet carpet, only on the passenger side floor, after the parked car was out in a short heavy rain downpour.
There was no wet trail indicating where the rain had got in. Ring any bells with anyone?

 

If you have a sunroof that could be your water source. The drains may be blocked. Two in front in door hinge area and two that drain from each rear wheel well. Leaves and other debris block them up. That's where my flooding came from.

 

Hello Pete,
thanks for the reply. Yes I think it must be a drainage problem. I don't have a sunroof.
Here is an answer I received from a UK VW forum. My Passat falls within the 97 to 05 age group:-

This is an extremely common problem on the 3B (97 to 2005) Passat, A4 and A6. In the plenum under the battery and servo there are two drain tubes, these become blocked with debris. The plenum chamber then fills up with water until it reaches the base of the heater intake pollen filter housing. If the filter has never been re-sealed with the new sealant cord (VW part number AKD 497 010 04 R10) then water will easily enter the heating system and pour into the passengers floor well. Even with the improved sealant, if the water gets high enough, it will still get in. The knock on effect of this is that the central locking system will start to act up as the control unit is under the passenger’s floor and even though it is in a supposed water proof box, it and the wiring suffer.