It's going somewhere. Refill it. Do NOT use mineral water or drinking water. Battery water or "tonic" is cheap and worth buying a crate of.
The key is to not have minerals in the system. Battery water is just water that is deionized or distilled, i.e. has little to no dissolved minerals. Minerals are bad for batteries just like they're bad for your cooling system.
My car is 32 years old and once I got the cooling system in order it has zero leaks and does not overheat. It has an 82°C mechanical thermostat and an electric radiator fan that turns on at 85°C.
Your cooling system's parts, quite basically, are:
- Engine and its coolant channels
- Thermostat valve (sets the minimum temperature)
- Hoses (lower radiator, upper radiator, bypass, etc. - all should be clean and leak-free with good quality clamps)
- Radiator (dissipates heat passively and in conjunction with fan)
- Radiator cap (regulates system pressure and relieves it when needed)
- Expansion tank (holds excess coolant that the system can use as needed)
- Cooling fan (sets the maximum temperature)