Evaporation, coolant runs through the hot motor and some of it turns to steam, other reasons may be that there is a small hole in the radiator or the hoses.
Normal causes are low coolant and an inoperative radiator fan.Normal causes are low coolant and an inoperative radiator fan.
Wind or air movement also promotes evaporation. Evaporation is also much faster in a vacuum.
Defective radiator caps or tubes
There are several causes that will reduce the water level in your car radiator. The most common cause is evaporation. Engine overheating will also reduce the radiator water level.
Check to coolant level in the coolant recovery tank and radiator when engine is cold
Possible trany cooler leak
Lack of coolant, lack of coolant flow, air in cooling system, lack of airflow over radiator,........
Broken or split hose Radiator leak heater core leak
I had this problem ....Head Gasket or radiator cap
Lack of coolant, faulty thermostat, radiator fan not working...
Could be, Restricted air flow through outside of radiator, Restricted coolant flow through inside of radiator, Faulty fan clutch, Missing or broken radiator fan shroud, Faulty thermostat, Low on coolant,