I opened a bottle of (moderately expensive) red that seemed to give off a fizzy sound. Could that be carbonation or was I imagining things?
Assuming it was your first bottle of the evening, not your third, I suspect your imagination was not playing tricks.
Fizz implies gas, and in wine, as you have wisely assumed, that almost always means carbon dioxide, a natural product of fermentation. Yeast consumes natural grape sugar to produce alcohol and C02.
For still table wines, you want the alcohol but not the gas. So, the gas is permitted to escape out the top of the (open) fermenting tank. When making quality sparkling wines (the kind with natural carbonation), producers intentionally trap C02 either in pressure-sealed tanks or in individual Champagne bottles with tight seals. Pressure ensures the C02 remains in a liquid state rather than expanding into its naturally volatile gaseous state – at least until the bottle is uncorked.
Your "still" red likely had undergone an unintentional second fermentation in bottle. Normally all yeast in the fermenting tank will have died by the time a dry wine is aged and bottled. But in rare cases, particularly with light reds that see only a brief maturation period before bottling, some yeast will have survived in a dormant state, making it through the bottling line. The yeast awaken at some point between the winery and your dining table and continue feeding off minute quantities of residual sugar left in the wine, producing, yes, carbon dioxide. When you open the bottle: pfffft!
I'm guessing your wine had been sealed with a screw cap, which offers a near-perfect seal and thus traps unwanted gases more easily than microscopically porous cork.
I think you'll find that the unwanted fizz in a "still" wine tends to occur more frequently in whites, notably styles that often come bottled with more residual sugar than reds. Riesling and gewürztraminer are classic examples.
The other week, for instance, I resealed a riesling that I had partly consumed, and when I pulled it from the fridge and unscrewed the cap a couple of days later, it gave off a telltale pffft. That's evidence that there was residual yeast in the bottle that had woken up at some point and begun gobbling small amounts of wine sugar.
Or maybe I was imagining things. Maybe it was the cat hissing at me for treating myself to an aperitif before filling her empty Whiskas bowl. Then again, probably not. I'm only imagining that I have a cat; I don't.
The Flavour Principle, by Lucy Waverman and Beppi Crosariol, was named best Canadian Food & Drinks Book in the 2014 Gourmand World Cookbook Awards. Published by HarperCollins.