It's definitely not the windscreen seal? There are products that "seek and seal" such leaks which can be very small.

In the late 1980s, my BMW went back to the main dealer no fewer than 12 times to find the leak. In the end, that's all it turned out to be after visit after visit and all manner of false solutions implemented.

Hey!

I am actually now dealing with the same problem as well. Today my brother helped me to take out the whole carpet so that we could see where the water is coming in from (we believe it is from beneath). Yet, we could not see anything, even though everything beneath the carpet was wet. Tomorrow I will be going to wash my car to one of those places where they can actually wash the car from below. Perhaps that will give us a clue.

I will keep you updated.

Best of luck in the meantime.

Exactly same problem with my 307. 2" deep puddle of water in the footwell. Probably forget the door seals (mine were replaced 3 times by the local Stockton Peugeot garage) - did not fix the problem. Difficult to imagine water getting in past the door seals and over the door thresh hold (anyhow could not see any water stains or wetness around the doors). Still mystified. Anyhow on another forum a member suggested checking under the bonnet and see if the external windscreen window drain is blocked - could it be running back into the car?

The problem has presented its self when the car has been stood so I have discounted a hole in the floor (and water spraying back under the car). Seems a common problem - judging by all the posts.

I have the same problem on my 508SW. I first thought it was the windscreen seal as there was a part of the seal coming away from the glass. Resealed this but made no difference. Water runs down from the left hand side into the drivers footwell when heavy rain or when I wash the car. Poured a bucket of water along the vents (not onto the windscreen or seal) and water runs down into the footwell. It appears to me that the drain vent is blocked ? But how to get at it to clear it is my next challenge.