Coolant level goes low but NO sign of any leak

Hello

A couple of days ago, in the evening since I could not see it clearly how much coolant level is there, I decided to open the cap to check the coolant level. At this time the engine was cold, turned off for over 20 hours and the car was parked on a flat surface.

There was still some pressure and all of a sudden the coolant level which was close to the "MIN" mark started to rise and came to just over "MAX" mark. I was not sure what had happened. Why did it move from MIN to MAX level mark?

Yesterday I was about to set off for another long drive and checked that there was NO coolant in the tank. When I opened the cap to top it up, there was a pressure build up and heard the sound as the car was trying to do gargles. Still no sign of any coolant, so I topped it up to the MAX level.

This afternoon I checked it again and it was below the MIN mark again (on a cold engine) and there is NO sign of any leakage. My driveway is all clear, the engine is all nice and clear, the car produces heat in few mins as usual and there is no other sign or warning.

Before that, it was always fine. The coolant stays at the MAX level all the time. I do check this almost every week/fortnight and never seen the coolant level is low.

I have now topped it up again and let's see what happens to it tomorrow.

Can someone please shed some light on this what is happening? Why the car has started to lose the coolant level all of a sudden since after opening it the very first time a few days ago.

Thanks in advance.