Diagnosed: Its the heat exchanger (heat core) that is "leaking" coolant. So ive ordered a new one, but its quite pricey. Im wondering maybe i could have gone aftermarket like this:
https://www.design911.co.uk/fu/prod135732/Heater-Matrix----Heat-Exchanger-Porsche-996---997---Boxster---Cayman/ for (labour+parts) of ~280USD instead of 600USD.