The idea of the expansion reservoir is to allow for expansion of the coolant when it heats up. When adding coolant in sealed systems with an expansion reservoir you are supposed to add the coolant mix to the expansion reservoir not the radiator. That being said, if the level in the reservoir continually drops its a real good sign there's a leak somewhere. The only way to stop it is to locate the leak and repair it.

You should keep the expansion tank filled between the "filled" and "low" marks all the time, with the recommend antifreeze/distilled water mixture (usually 50/50 - or buy the 'ready to use kind) all year long. As long as there is fluid in that expansion tank, there should be no need to open the radeator cap to fill it.

If you are loosing fluid regularrly you should find the leak and have it fixed. These things don't usually get better on their own.

The purpose of the coolant recovery tank it to allow the expanding anti-freeze a place to go besides on the ground - so as it heats and expands the fluid is dumped into the expansion reservoir. As the car radiator cools it is sucked back into the radiator keeping if full all the time. So keeping the reservoir at the correct antifreeze level insures your radiator is always full.

This is a closed system so there is no reason under correct/normal operation you would need to add antifreeze.....you have a problem -

The least of your worries is an external leak (look over and under the engine (especially where the coolant hoses connect and all around the radiator looking for telltale signs of an antifreeze leak. (color of the antifreeze you are using will dry the same color so usually visible. Also check inside the car under the dash where the heater core lives - -although if it were leaking you would smell hot anti-freeze every time the car came up to operating temperature.

You can buy UV Dyes to add and a special pair of glasses and light which will help you find an external leak

If you have an internal leak (also known as a blown head gasket) the leaking antifreeze will either be going into the engine (combustion system) or the oil.

If you exhaust steam excessively and has a sweet-ish odor - problem if not then check your engine oil - if it is milky and overfilled that is another sign of a blow head gasket.

I would suggest either taking to a qualified mechanic or doing the UV test yourself first before jumping to conclusion.

Can I have more info on this?

i see my expansion cab decreasing about 500ml every month.

is it mean coolant in radiator decreasing too ?

when o open radiator, there is no space for filling coolont liquid.

is there any benefit add antifreeze to expansion cab ?

will it go through my system ?