If the radiator itself remains full and the system drinks 500ml on each startup then there is an active leak which is exacerbated when engine is warm,
The leaks can be from the hose pipes, the metal intake pipe, the waterpump or welch plugs, and its almost guaranteed to be leaking if the engine was cooled with plain water only.
Get a pressure tester and test it at 15 psi to see the leaks.