Problem I'm losing coolant


I put the car up for sale again. Last week someone came to see it and on this occasion he drew my attention to the fact that the coolant level is low. Initially, I put it to the fact that the engine was hot and probably because of the heat the level also changes.

However, the idea remained in my mind and I checked after a few hours, when the engine had cooled down. The level had increased a little, but it was still below the minimum.

I opened the cap of the expansion vessel, it blew a little, a sign that there was still pressure, but it still did not rise to the level.

I got dizzy. I immediately checked the oil level, knowing that BKDs have problems with cracked cylinder heads. Fortunately the oil level is normal. There are no traces of mud or dubious things.

I topped up with antifreeze when cold and it seems to me that a part has disappeared again. Where could it flow? I park the car in the basement where it is dry and I have not seen anything leaking. I looked, as much as possible and through the engine, and nothing suspicious can be seen.

What else should I look for? Could I take it to Porsche to have it checked? Especially since I also have a voucher for mechanical work that I could use. Is there a test?