Coolant loss is not normal in any circumstance. The only way for water loss is through a leak in a hose, leak from a gasket, leak from a radiator, heater core, faulty radiator cap.
Once the water is in there it should stay. Water vapor is not lost into the atmosphere, it condenses back to liquid within the cooling system. The vapor pressure of water is raised due to the system being a closed system, under pressure, and the thermostat opening below the boiling point of water. (212 degrees)
If your level is dropping you have a leak or you are overheating and the system is letting off the pressure.
Make sure you do a pressure test over time. It would take a very long time to see a small leak in a pressure test.