Salam,
The coolant level should be checked when the car is warm because coolant expands and if you pour coolant to the brim of the reservoir tank and when the car gets warm the coolant will overflow from the reservoir tank, so check the levels when warm, if the coolant level is decreasing there is some leak or major problem with the water body of the cooling system of car.
coolant level of a new car SHOULD NEVER DECREASE even a single drop.
my fathers city aspire 1.5 prosmatec is 85,000 Km driven in 2 years and the coolant level is still the same as when we bought the brand new car.