Not sure that this adds to the numerous threads about water under passenger carpet but here goes. Had the car just over a year now and when I got it spent a whole weekend servicing, cleaning, polishing etc and noticed that the underside of the carpet underneath the seat felt wet ie where the two parts of the carpet overlap. Managed to slide a towel under the gap and been changing it regularly, always coming out fairly wet. Anyway in my ownership car has not been used in the wet nor wet washed so I cannot have added any further water.
Decided today to track down what could be happening. Hood drain holes not blocked but I still cleaned out and water ran freely out the drain hole. Removed door card to check membrane and it looked new (perhaps it is) and seal intact so cannot be getting in from there. Next was to check the scuttle underneath the windscreen to see if build up of gunge could be blocking drain holes, nothing, so poured water down just to check and ran freely out.
Next step was to remove the passenger seat and trims to gain access under the carpet. Found a 5p piece under the seat - not the pounds that others have found but still money is money. Sponge was very wet and took some time to dry out - good job today was nice and sunny. Couldnt see anything obvious so reinstated everything and not sure where to look next. It did occur to me that a previous owner may have had a problem around the three usual problem places, resolved it but didnt dry out under the carpet. I guess I will have to wait to see whether water comes back and check again next summer.