This can only happen when and if your coolant hoses can expand beyond its normal size aka it is asking for $$ already
By expanding, it can take more coolant volume, thus the reservoir bottle coolant level goes down.
If heater core is the one you suspect, its total hose length is quite long if based on my engine.

This also means your engine has coolant shut off valve towards the heater core HVAC in the cabin.

Air bubble trapped in HVAC heater core and its related hoses.....can also contribute some

The long story short is Mercedes coolant system leaks either slowly (monthly) or rapidly (weekly) or productively (daily).
MB coolant level must be maintained at regular intervals with matching chemistry.

>>Do not open HOT system under pressure (200°F shower causes burns!) - When you say "pressure is released" is a bit worrisome... cold system has zero pressure: ok!

-- Check coolant level cold only... just like with tires control!

-- Tell us how you car is driven as this impacts heat production by engine and release by radiator.

-- What size leak... S-M-L?


> Here the questions may be :
-- what leaks ?
-- what does not leak??

I don't see no leaks, When i open the coolant cork it releases like some air psshing sound and the coolant levels rise back to normal.

All good: back to normal is where coolant level should be.

https://vimeo.com/manage/videos/1031844024 a small video of the issue this is opened after drive ,after initial release and fill it slowely fills rest up allso.

This confirms your coolant tank is low!!

-- DO NOT OPEN your coolant circuit with HOT engine temperatures: dangerous burn!

-- Do top off coolant tank with COLD ENGINE ONLY using compatible MB coolant.

-- Locate LEAK where coolant is going....

-- Coolant does not evaporate under sealed pressure. You have a leak.

It all slowelly fills back up after air is out.

safety first

MB Specialist can pressure test your coolant circuit to find leak at failed radiator crimps or engine oil cooler.