Car is losing engine coolant, its not leaking it. If you don't replace about evey other day, car over heats when setting still.
Losing Engine Coolant causes engine to over heat. on 2001 BMW 330Ci
If the coolant is not visibly leaking, you could have a blown headgasket. If this is the case, a shop should be able to detect hydrocarbons in the cooling system.