For a while my VW was leaking a little coolant. Naturally, it uses a "special kind", so I bought the expensive bottle and topped it off a couple of times. The reservoir was perfectly easy to see, but it was indeed hard to get the level just right because it would fluctuate. Anyway, the problem "went away", and a couple of years later while it was in the shop for something else, they finally found and replaced a hose or something and it's been fine since.
Cars. Go figure.