Expansion Tank Issues: Pressure & Troubleshooting

Hello everyone,
It's me again. I think my head gasket is starting to fail at 203,000 km on my AFN engine. The only thing I've noticed is a slight coolant leak. Over 1000 km, it's maybe only 5 mm in the coolant reservoir. When the car has been driven for a while (50 km), if I open the coolant reservoir cap, there's a brief hiss and a little water spurts out. Is that normal? I would expect a little pressure when the water is hot. The water isn't discolored, and it doesn't smell like exhaust. Also, I don't see any white smoke when I start the car.
Do I now potentially have a cylinder head gasket (ZKD) failure or a leak somewhere? I apparently couldn't detect any leaks.

Are there any reliable tests to determine this?

Greetings.
Reuselkopp.

Hi,

Is it normal for it to hiss and bubble if opened carelessly while it's hot? It's a closed pressure system, and the water can easily exceed 100°C (212°F) after the boiler process, especially with these weather conditions.

When you look at the container with a cold engine, the level might appear different than when everything has expanded due to heat and pressure. This could explain the 5 mm difference.
Just keep observing it.

'With my AFN, it was around 160,000 km when it reached that point. I also had a clean engine.'
Unnoticeable exhaust emissions. However, there's a latent coolant leak, which only occurs when the oil temperature is above 90°C. The test that provided clarity was the emissions test (AU); it sounds silly, but it's true. The opacity is then too high to pass the test.

Is there an indicator in the workshops that can be easily placed in the balancing tank?
dives, and immediately knows what's going on?