That is the original cap. As you have noticed, it will leave a mist on the underside of the trunk lid. It has been updated and the latest model number ends in .04. Independent of the source of your leak, replace the cap.
You also need to look and see if your vent purge is leaking. Pull the Oil cap and the Radiator cap off. There is a thin black plastic plate covering the vent purge at the bottom of that service area. It is not obvious that it comes off.
The trunk coolant tanks are notorious for leaking. I have not had the pleasure yet, but I understand that they are quite challenging to X/O. The general consensus is to use OEM tanks only. The aftermarket tanks are generally poor quality.
The site search function is your friend. Also,
Pelican Parts - Porsche, BMW, MINI, Mercedes, Audi, Volkswagen, Saab, Volvo - Auto Parts and Accessories has plenty of DIY articles with pictures
