Cooling system losing water

Leaking water. No visible leakage. I can fill up the car to the "F" mark and drive to my destination(e.G. 40km), and check immediately after arriving, then the water-level is still the same. However around 2-3hours later when the car has cooled down, then the water level will drop. On average the water level will drop with around 6-7cm per day, but the last 2 cases the whole bottle became empty.

Hyundai checked the tubes and did 2 pressure tests, nothing is wrong. Replaced the radiator cap. Still do not know what is wrong.

The temperature meter has spiked once up to almost in the red, but not again. It's just below the half mark on average.

I do not use the aircon often(Perhaps 10min at a time, 3times a week). I do use the heater quite often, average temperature setting of 24-28degrees.