Low Coolant Level Mystery: No Visible Leaks, Head Gasket Damage? Fluid Decrease Pattern

Hello!
I have a rather unusual problem with the coolant in my car. I looked at all the visible installations of the circuit and did not find a fresh leak (there are many traces of old, but after cleaning it does not leak) and the fluid decreases anyway. It is interesting that when I add the liquid to the level of the marker, for a week the fluid will fall to the level of 1 cm from the bottom and then gradually over two years the tank has dried completely (I purposely did this test). When I added it at the beginning, the level always went down to 1cm, and when I stopped adding it, this 1cm came down exactly after 22 months. Last Wednesday I added to the marker again (6 days ago) it again went down to this 1cm and remains stable. No visible leaks.
Could this point to a damaged head gasket or the head itself? There is no water in oil or bubbles in the system, but half a liter of liquid disappears somewhere in a few days and it is not visible on the system.
The problem still exists, despite changing the timing six months ago (apparently the old liquid pump has leaked). By eye, the head is (except for the coolant tank itself) the highest element of the system.
Thank you in advance for any suggestions.
Regards Michał.

A hole in the tank just above the top of the fluid. It will escape to its height and then it will hold.

Pour liquymoly snot does not clog the system and invisible leaks are usually removed.

Przjaciel wrote:

It is interesting that when I add the fluid to the level of the marker, it will drop to a level of 1cm from the bottom for a week and then gradually over two years the tank has dried completely (I purposely did this test)

I don't know what kind of car it is, but since you didn't run away from the liquid from the pipes (the tank fluttered all that was above the pipe stub), then see what is at the height at which the liquid stopped running.

I would like to join the system, somewhere near the engine, a transparent rubber tube (open, attached to e.g. a mask) and without driving / firing saw at what height the fluid column will be after a week ...

I think that if it was a head gasket, the symptoms would be different. Either sucks the liquid like a vampire or it gets aerated. Here we have a small leak. Maybe from a heater that can't be seen.

But he escapes to a certain level where there is probably a hole and then holds the level. This is what you are looking for on a cold engine, adding pressure to the system so about half a bar and any leaks come out. You just need to sacrifice the tank cap to put the valve on the car wheel.

Even simpler. Pour fluid to the required level. Close the tank and squeeze the upper radiator hose as long as it is with two hands. You will know something initially.

In fact, I forgot to write what kind of car it is. This is an Opel Astra F with 1.7D 44kW GM engine. The expansion tank is certainly fine, because recently when I ran the power cable to Car-Audio I dismantled it without disconnecting from the installation. It is in good condition, slightly yellow, clean. The wheel arch over which it hangs is also clean, without any trace of these characteristic fluid stains (I only fill with petrygo).
At the level that the fluid level stabilizes are around the valve cover / half the head height (hard to say clearly), the thermostat (two years ago cleaned and only dusty since then) and the top of the radiator (this one is also clean).
I have already thought about the fact that the leakage may be lower, but (on my logic) with such a rapid loss, the fluid would escape until its level equals the level of the hole, which would result in a significant loss of fluid.

Try to pressurize the system as the mouth wrote. I forgot to propose it earlier. The method is very good, preferably in a quiet peaceful place and do not top up for the occasion. Maybe you will hear something.

What about the tank cap? I probably had the same, I wanted more, 3/4 tank, and a car 1/4, below the minimum. In a while it passed, but it seems to me that the cap did not hold pressure with the valves and the liquid evaporated. There were marks, but the defect was faster.

The plug is easy to check after driving as the whole system is warmed up. Hand on the snake and the other on the cork. When loosening the cork, it should hiss and the hose will soften.

Another thing is that the valve in the plug withstands too high pressure. Correctly it should release around 1 bar depending on the car model.

Perhaps the latter, I had a breakdown, the cable burst and after replacement it was necessary to flood the system and let it bleed. So more fluid that the car did not want to stabilize in the middle of the tank. The cork has dropped it too much three times, going below the minimum. Washing and blowing the cork improved the situation.