Welcome, Srlc!
Hmm, that's a poser! I have seen snails eat sage leaves and small twigs, but they usually didn't confine themselves to that. Some caterpillars are pretty choosy, but you should have noticed smaller damage first, as they grew up. Is the garden completely open to the wild?

I wonder if you have deer nearby.

Thanks for considering the question! The garden is a small suburban one, fenced in, but with access for squirrels, birds, hedgehogs, urban foxes. No deer though! And no caterpillars on the other plants, nor that I noticed on the sage plants while they still existed.

Neither squirrels nor foxes would eat the sage and birds would have pecked pieces out of the leaves. In face sage is one of the plants that even deer and rabbits are supposed not to like - the oil in the leaves I believe. I know you say you did not see any but I suspect it was caterpillars they can strip a bush if they put their minds to it.

Since they work at night, you don't often catch them in the act.

Very true Tugbrethil.

Thanks again - but would caterpillars (even invisible caterpillars) actually demolish the bushes once they'd eaten the leaves? "Demolish" as in eliminate entirely from the herb patch, twigs, trunk and all! And no caterpillars on any other herb (thyme, lemon verbena, oregano, marjoram, coriander....)

Once caterpillars get big enough and old enough, they'll start in on the green twigs, but not the woody brown ones. Like I said, some species of caterpillars can be remarkably choosy, only eating one species of plant. Only a few kinds are generalist enough to go for several species of plants. Most other chewing garden pests, such as beetles or grasshoppers, aren't so fussy.