Got the same problem on my 5 door. Have checked the pollen filter as suggested in other threads, and that is okay. Rear seats and spare wheel are bone dry. After it's rained, there's a few tell-tale rain drops on the inside of the front passenger door near the puddle light and the plastic trim that covers the passenger seat slide rail. And the rear passenger footwell is damp. I'm almost certain it's that inner door seal that others have mentioned, but haven't got round to removing the doorcard yet. Just been putting some kitchen towel in the gap under the puddle light to soak it up for now until I get round to sorting it out properly.