Thank you for the replies!
I'll try to address the specifics.
I put on the newest cap a couple of weeks ago but it didn't change the behavior. There is no moisture or signs of condensation in the trunk. I checked under the carpet and it's dry. No residue visible at least in the easy to check places. The shop had checked it both cold and hot. radiators have been checked, but I'm not sure about the heater core. All lines were checked and one loose clamp was found. Replacing it unfortunately didn't fix the issue.
I don't think the bottle has been removed to be checked out. The system was purged at least once. Water pump is fairly new.
Yes, using the Porsche approved coolant.
Just a maddeningly evasive habit here.