Water leak, there renowned for it. The main suspects some as mentioned above are, sunroof if you’ve got one, battery chamber drainage blocked causing the water to come back through the pollen filter, debris around the bonnet hinges causing water to build up round the pollen filter and get in, dicky door seals, etc etc have a look on vwaudifourum for more. Tends to accumulate round the nearside passenger foot well, you may need to lift the carpet as sometimes if the leak is slight the carpet seems dry on top, but once you get underneath you find its damp.