Nope no trees, unless the previous owner did, only thing I can think of was the amount of water I've had to drive through this past few days (North East) - I'll keep an eye on it, the driver side was dry incidentally.
I've literally just had the same problem with my mums car, she had damp on floor and had water sloshing about in doors etc if you turned. It was blocked drains as Karl said. So I'd make sure they're unblocked first
Had the same issue, aircon pipe was not fitted properly and was dripping fluid INSIDE the car instead of outside. Dealer had to strip whole interior and steam clean all the carpets, all done under warranty.