How to find leak in pressurised CH system?is it possible?

[John D v2.0]

But whether it’s linear or not, I don’t think you can get away from saying that if 3 litre water input raises the pressure 0.5 bar, water outflow to cause a drop of 0.5 bar is of the same order

You're forgetting that the air can't expand freely as water is lost. The air is constrained to the expansion vessel.
Everything works per your intuition, until the moment the air needs to expand outside of the ev. At that point it can't, so the pressure in the ev remains at the charge pressure and as soon as more water is lost the water side pressure drops to however many metres of water above.