I have confusion as to why people need to top off their coolants from time to time. How is that a phenomenon if the cooling system is supposed to be a closed loop? I understand that if there were leaks, then losing coolant is apparent, but if there are no leaks, how is it possible? I thought that if it was a closed loop, you should not lose a single drop of coolant.
It's not a totally closed system 100% of the time on most cars that I've seen, but let me explain in detail.
The overflow vents to outside air and you can lose some to evaporation over time. The way that could happen is that on most cars the reserve overflow tank is not pressurized. What happens is if the pressure builds up too high in the radiator cap, it opens and releases coolant to the reserve tank. At that point, while the radiator cap is open there is some pressure to the reserve tank and could be a lot if the car is overheating, but at that point, it's also not a totally 100% closed system either, albeit temporarily, as it's letting out coolant to the reserve tank.
In my thinking this could account for a small amount of coolant loss over a years time.
Water pumps can weep coolant, but not always present or easy to see.
A hose that appears fine, yet under pressure could weep out small amounts of coolant, like from a clamp that isn't fully tightened.
I recently had this happen, a "T" connector was breaking right at one of the connection points and was leaking a small amount of coolant. I could smell it, but not find it, until it finally gave out.
Also, if you have an older radiator cap that may need replacing, it could be slowly losing coolant.
So my point is you have to factor in the age of the car and the fact that lots of cars have plastic parts in them now, even in the cooling system like I said, which I think is a bad idea.
There must be a leak somewhere. There are quite a few spots where you can have small, almost unnoticeable, leaks.
Every hose connection in the cooling system is a potential leak as are all the gaskets. There could also be leakage at the pump shaft and at any valve. None needs to be more than a tiny bit to add up over a year or two.
On my Jetta there appears to be maybe 200 ml between the low and high marks on the reservoir - 0.5 ml per day would have me down almost 400 ml in a couple of years. I don’t think a leak of that size would be noticeable at all - especially if it were at the pump and a couple of hose connections.
It might be worth checking your head gasket, the testing kits are usually pretty cheap and easy to use. The only reason for saying this is that head gasket leaks aren't always very obvious, in some instances, they wont make noticeable smoke out the exhaust.
Other than the (in most cases kinda unlikely) head gasket leak, some older cooling systems can loose some fluid after a while where the water in the fluid evaporates out of old seals and loose radiator/ res caps.
It will be worth covering all your bases with it. But at any rate good luck :)