I had a situation where the rear driver-side carpet was sopping wet after it rained. Couldn't figure out where the water was coming from, although I did used to park on an inclined driveway with the car facing up (?)
Anyway, at the time the car was still under warranty, so I brought it to a dealer, and they supposedly found some leak, and tried to fix it with some sort of anaerobic sealer. I had the problem again though, but it only seems to happen on a steep incline, facing up.
When I was at the dealer, someone said they had the same problem with their car - I believe with a Bonne.
Now, I too, am afraid to look under the carpet.