If coolant is needing to be refilled, where is it going?

If a car is not overheating, yet coolant needs to be topped off periodically, what are the possibilities?

A leak somewhere? (Hose, etc)
It’s a closed system, so where else can it be going?
But, since it’s a pressurized system, wouldn’t that mean overheat?

Or does it have to mean there is a head gasket leak?

coolant needs to be topped off periodically

How often (miles and months) do you need to top it off? 
How long have you had the problem? 

What are the driving conditions where you are driving?

let’s not forget leaking intake gaskets . . . I’m thinking domestic V6 and V8 models

An intermittent fuel pump leak is possible.
I had one that only leaked for about a minute each warm-up.

Many cars have plastic radiators and surge tanks. Plastic will crack over time and have very small coolant leakage which is almost impossible to see.

So, it’s a breach in the system, and it’s either leaking outside from a crack, or leaking into the engine? It’s not like it’s just evaporating, correct?

I dunno, In four years I’ve added two cups of coolant. I was concerned where it went a few months ago but found no evidence of anything leaking. On my new car I also added a cup or two in a year or two. Given that manufacturers put stop leak in the systems at the factory, I don’t know if one should be terribly concerned about having to add a little coolant from time to time. Every week, then start looking for a leak. Where does it go? I don’t know. It is half water though and water does evaporate. Hoses sometimes seep a little, and so on.

If you only have to top off every couple of weeks, then the leak can be very difficult to find, even if it is an external leak. Eventually it will show itself.

Well, by the laws of physics, it has to be going somewhere. It depends on how much you need to add. On my Corolla I need to add about a pint every year. Like you, I’ve always wondered where it goes. My theory is that it escapes as vapor from the radiator-cap/coolant-tank system when the engine is hot. Rubber hoses and seals etc develop tiny holes and cracks and could leak pressurized vapor it seems like. The only confirmation my theory has any merit is that I am able to smell a very faint anti-freeze smell if I put my nose right over the radiator cap when I stop the car and the engine is up to full temp. I can’t smell it anywhere else, other than right over the radiator cap.

I recently went on a 5300 mile road trip and my '06 Matrix used about a cup of oil and a cup of coolant.
There’s no sign of an external leak and there’s only 41k miles on the odo now, so I think the water pump is still tight.
Doesn’t use any measurable amount of coolant in the mostly city driving I normally do.

Do you ever smell an antifreeze odor around the car. You’ll usually smell a coolant leak that’s to small to see. Also, the green or orange dye in antifreeze stays behind and will sometimes leave a colored stain in the area of the leak.
If it’s the heater core, you’ll not only smell it while you drive, but you won’t be able to drive with out a window open or the AC running without your windshield fogging over.