Can the Deer get to your plants from the yard and browse between the rails on the deck? are the leaves gone on the side facing the yard? I'm stumped! sure sounds like deer. I hope you find the culprit and let us know.

Could be anything. Assorted bugs and creepy-crawlies know what's good for them just as we do. I suggest a garlic spray which is a good treatment for many creatures.

Try pepper wax and if it's still gone, you know it's definitely not a mammal

Pepper wax sprayed on edible foliage can kill the foliage or change the taste to something other than good.

Rabbits!

The patio/deck is a small area 10x10' next to the house & surrounded by a 5' brick wall and 6' shrubs. The deck base itself is like 1' off the ground with 1/2"? between the redwood panels. The gate is always closed.
The sage is up on a 3' table but nowhere near the wall. I can't see deer or rabbits getting anywhere near & if they did I think they would prefer the lettuce or edible flowers. I'm starting to wonder if someone is just messing with me at this point! ;)

I noticed this about my sage this year, too! Whatever it is doesn't even eat the leaves, it just pulls them off and there they sit in the dirt. Like someone plucked the leaves and dropped them in the dirt. No deer here, no squirrel. Maybe some rabbits, but mostly birds.

Same thing happened to my dead dicentra (bleeding heart). It was dead but with branches and something just ate all the branches off. Didn't even take them, just left them there.

What the heck is right???!!?!?

Well, if not Deer! and I doubt Leaf cutter ants? in these parts? nope! didn't think so...Just what could it be!!!

This is just a wild guess, but leaf-cutter bees? I have lost leaves off trees, shrubs and basil to leaf cutting bees. They take the leaf off the plant, roll it up and use it somewhere for a incubator for their young.
I've seen them cut borage leaves and decide they don't want them (I think it is the fuzziness, somewhat like sage, that turns them off) and leave them at the base of the plant.
Can you put a hairnet over your sage? I'm not kidding--a fine mesh net of some sort over the plant will discourage cutting insects. Not decorative, but effective!

A couple years ago basil leaves, only from the basil plant while everything else was left untouched, were disappearing. Turns out it was a chipmunk who apparently liked the taste of basil. (I caught him sitting in the pot chomping on a leaf he had just pulled.) I had never seen or heard of chipmunks eating basil before. Maybe you have one near your patio that likes the taste of sage??

What the HECK would eat sage?!

Me. :)

Check with your local cooperative extension to find out the main pests in your area. It varies quite a bit depending upon your location.

For my $.02, if you don't see any caterpillar poop beneath the plants, I'd lean towards to the garden pesty mammals being the culprits.

FataMorgana

I have had sparrows pluck pieces of tansy and fly off with it, presumably to deter pests in their nests.

It is possible a bird thought the same use could be made of sage.

Chipmunks are capable of anything, though. I know that one demolished a globe basil of mine.

Same thing happened to my sage! My plant was very full and beautiful at one point. After I planted it, i would come out to plucked leaves around the plant every morning no bite marks just perfect little leaves. I thought it was squirrels but turns out it was a little brown sparrow coming by. I watched it land on the planter and pull off the Leaves. it didn’t take any of the leaves it just sat there plucking them like it was fun until i shooed it away. I purchased an owl statue, hopefully it will stop the birds from coming near my balcony