Hi, I'm not trying to hijack your thread (although I'm desparate!).... Sorry this is a long rant.
I have a similar problem, Clio 3 2012, 3 door,
very low mileage, no sunroof always been dry suddenly found underside of passenger seat and behind passenger seat saturated. The car doesn't have a sunroof and has been dry from new (I'm 2nd owner but I bought from someone I knew well.)
Obviously I went online to investigate hence why I'm here.
I did find build up of water in the scuttle on that side and cleared the drain, but I couldn't understand why it wasn't wet (maybe a bit damp- hard to say) forward of the passenger seat, which I would have thought would be the case if it was overflowing from the scuttle? IS THAT THE CASE? Where does scuttle water go when it can't drain?
A visual investigation shows all plugs present underneath. I was praying I'd find something amiss because I drove home one night through some very heavily wet roads, and noticed it after that, but that could be complete coincidence.
Looking at the area that was affected it seemed to be localised around the passenger door, but as has been said many times water finds the lowest point so it's very hard to find the ingress point.
The weather hasn't been suitable to strip it down, and I've now wrecked the foam under the seat trying to squeeze out all the water, used £££ paper towels, not to mention that when temp drops outside condensation on inside is unbearable (especially when it freezes!)
In the meanwhile I'm guessing it's it's coming out of the passenger door and working it's way under the perfect door seals! (They are perfect, I've examined them very carefully) That said, so far I couldn't see any evidence of water below the level of the door card on the paintwork below it, but I have to wait for drier weather and a mate who can hose while I look carefully. I would have thought that any water coming out of the door should drain onto the cills and to the outside, unless, like the OP it's coming out of the bottom of the door card, which is effectively just about on the inside of the car.
In the meanwhile to prove the point (or not) I cut a long strip of polythene and laid it over the door seal and cill (see photo), on the grounds that if it is coming from the door that will stop it going back into the car.
So far (and it's early days) it doesn't seem to be getting wetter, but since it's not completely dry yet and the foam was sodden I can't be sure if that's stopped it. This is going to take a long time to investigate/fix.
When either door is opened after rain, a lot of water pours out of the bottom of the doors, less so on the passenger side than the drivers side (maybe a clue in there as the difference could be what's ending up in the car!). There is some sort of mechanism which retains the water in the door until it's opened, but like the OP I can't understand this on 2 counts: 1.
Why not just let it drain out over the sill, why retain it in the door? Does anyone know? 2. What happens if the car is stood for a long time in the rain (or if it's being washed), does the water just continue to rise in the doors??
It was suggested on another forum that there was a lot of water in the doors because of the seals on the bottom of the windows, so i cleaned them but it's made no difference to the the amount of water the doors collect, although this is an aside as it's always done this even when it was dry inside.
I'll try to report back when I know more, but any suggestions or info is welcome.