What causes a coolant leak to happen and then suddenly go away on it's own?

To understand where the leak is coming from, you really need to observe the leak and see what part of the engine/vehicle the leak is happening ... yah, that's sort of round logic, eh? My main point is, you don't say in your question where you observed the coolant coming from, just that it was using a lot of coolant. If it's the radiator hose, so be it. If it actually is the radiator hose, I'd think the mechanic would have tried to replace it for you (at a cost, of course). If it is the water pump, it will usually leak from either a weep hole (or some call it a "pee hole") which will be at the bottom of the casing, or from where the seal shaft if the bushing wears out where the impeller shaft goes through the casing.

It could possibly be the water pump, but realize the pump and the freeze plugs are in very different parts of the engine (in most cases). What we call freeze plugs are actually core plugs, meaning they are holes left in the block during casting where the sand core is held in place to allow for a proper flow of molten metal. They double as freeze plugs because it's convenient to have them there and it works. You shouldn't need freeze plugs up by the water pump because you already have holes there which are plugged by the water pump and serve the same core plug purpose.

Something about mechanical things is, they don't usually fix themselves (Note: I put usually because I hate to use absolutes like always or never ... in this case I'm tempted). Leaks just don't quit leaking (in almost ALL cases). Broken things don't magically heal. A coolant leak may stop leaking, but you can be sure it will be back, bad as ever when it decides to do it ... and most likely it will be at a time when it will be completely inconvenient to you, have no doubt (think: Murphey).