It certainly COULD be a leaking cylinder head gasket. A small leak will
not reduce engine performance sufficiently to be noticable.
I had a 1992 Chevrolet Cavalier that was losing coolant due to a leaking
cylinder head gasket, and I only had it repaired once the leakage was
bad enough that it was affecting the car's starting and idling.
Nowadays most cars have aluminum cylinder heads to save weight, but
these are notorious for warping after 100 thousand miles or so and the
cylinder head has to be removed, rebuilt and replaced.
But, it would check the simple and cheap stuff first. I would replace
the radiator cap. After repeated heating and cooling, the spring
tension in the radiator cap can get weak so that your coolant may very
well be escaping in the form of steam and ethylene glycol vapours from a
radiator cap that's got a weak spring in it. If replacing the radiator
cap doesn't resolve the problem, then I'd have the cooling system
pressure tested to see if it's holding pressure or not. And, if it's
not holding pressure, and the coolant isn't leaking OUT of the engine,
then it's leaking IN TO the engine.
Check your oil dipstick. If antifreeze is leaking into the crank case,
it'll muck up the oil. If the oil on your dip stick looks normal, then
at least you can be confident it's not leaking into the crank case.