Some are sunken, others aren't. My first thought was fungus or perhaps exposure to cold. I'm just perplexed because it's only on the underside. I can't even see the sunken spots from the top. No sign of damage until you look under the leaves, thus why I never saw it until today when I moved it inside.
Shoot for the stars in everything you do, that way should you miss you will at least land in the heavens.