Thank you guys for your input, sorry for my late reply.

Gary (Thanks !) gave me a clue to identify where's the smelly source, guess what, it turned out to be the Shevron Dash Mat (DM1196) I added. That also explained why other Polo owners haven't encountered similar problem like mine.

The dash mat was bought from Auto One last December (was on special, but no stock for Polo, so it's a special order) now I will try what they can do.