What is wrong when the coolant keeps disappearing but not visibly leaking?

Answer 1

First, if you only look after it has disappeared, there is no coolant left to be leaking

Second, coolant will often leak only under pressure, when it expands from being hot. This often happens when the car is moving, so cannot be observed, or escapes as steam. These leaks can be from small cracks in hoses, often under the clips; from pinholes in the radiator body; from under the pressure cap if the seal is badly worn.

If some coolant is already missing, the remainder will heat up faster, setting up a viscious cylcle.

In vehicles with an overflow bottle, overheated water goes to the bottle, to be sucked back later by the reduced pressure as the water cools again. In extreme overheating (boiling) the coolant can overflow the bottle, so less is available for sucking back later. Also if the hose to the bottle is cracked or loose, the reduced pressure will suck in air instead of water from the bottle. This leads to a situation where the radiator is quite low (so susceptible to fast overheating) while the overflow bottle contains water, so the user thinks there is enough water and never refills it.

Coolant can also leak into the engine block, but this will not "keep" happening, as quite a small amount will cause the engine to stop functioning.