Radiator: Finned aluminium panel placed in the frontal part, inside of the engine bay.
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Inside of it flows the hot refrigerant. Its purpose is to allow the air at room temperature between the fins to cool the refrigerant. It has 2 thick hoses, one in the opening (upper part) and another in the exit (lower part).
I'm noticing a similar issue with my 2012 sedan, the coolant system has no leaks but it seems to always want to stay at the low level line on the reservoir. When I top it off to the upper fill line, it consumes it back down to the low level. But I've never seen it go below the lower level.