I have the same problem - recently bought a 2005 grand scenic, and at first thought it was excess water from the showroom having cleaned it. After painstakingly sponging out the water (through fear of it smelling), it has all came back!
I don't have a sunroof, so can't be that.
Have had the carpet up and it seems that a pipe (possibly connected to the air con?) runs around the little cubby holes, so I wonder whenther it couls be from that?
I don't think that it is the door or window seals, because there are no water marks higher up.
Any ideas anyone?
Chris
August 2008