You chose an Arosa over an MX5?! From a practical point of view, I can see why, but MX5's are a hoot to drive and own, constant smiles everywhere you go (I'm on my second).
Anyway regarding your stripe, it just looks like the previous owner added it to the car. If I'm mistaken however, I'm sure someone will be along to correct me!
Edit to add: I own a Lupo as well as an MX5 btw.