Hi everyone! I'll try to be as brief as possible:

Gasoline engine, code AUS. Standard transmission, 190.000 km
- 3w ago started leaking coolant when parked, on the back-left of the engine (rear corner, timing belt side)
- my mechanic can't figure out where it originates. It's below the intake runners, behind the oil separator, but I think we'd have to take the runners off to actually see it. Did a pressure test of the cooling system, it passed. Water pump is not leaking. Hoses are all good. Says to come back when it gets worse.
- after he worked on it I saw the coolant level low. I topped it with something which says is G12 compatible as required by VW.
- after that, it stopped leaking until 2 days ago.
- I opened the expansion tank and now I have bubbles / white foam on the surface of the liquid.

Other details:
- the small puddle on the ground only forms when it is parked for at least a day. Doesn't drip to the ground when the engine is running, in fact the wet stream down the engine dries out when it's running.
- engine runs perfectly, no weird noises, no overheating at all
- no milky stuff in the oil, doesn't lose oil
- heat in the cabin works
- no white smoke
- from the pictures I see online the intake manifold gasket on this doesn't seal cooling channels
- this car has no separate radiator cap

Any ideas? Head gasket? Cracked head or block? To me some clues match, but some don't.
Thank you very much for your attention!

P.S. I stuck a small camera down there and took some pictures, I can link those if anyone is kind enough to look at them, but I don't know if I am allowed to post links