A week ago, a two by four was set to brace one heavily ladened branch of an apple espaliered to the wall of the garden’s shed. I was concerned that the bent branch might break, but I need not worry any longer. The apples are gone.
I suspect deer, ever present but mostly deterred by a repellent, or squirrels, that seemingly defy gravity for a morsel of food. I don’t recall if the apple was sprayed with deer repellent, but the next application is a week or two overdue so they are possible suspects. Still, no evidence was left behind so I’m betting on squirrels.
One, and I assume it is only one, climbs a Japanese maple just outside our kitchen window, follows branches that bend severely to snatch a seed near the branch tips. This seems a great effort for little reward, and where is this squirrel when hundreds of seeds are falling from other maples? Seeing this, grabbing fruit from the low branched apple is easy, so the culprit seems clear.
Ramblin' through Dave's Garden 
Did you find any apple scraps on the ground? Both deer & squirrels will sometimes leave dregs. Something else that enjoys apples – especially easily reachable ones – bear.
No evidence left behind, which I think points to squirrels. I’ve seen a bear in the neighborhood, but it would squash other plants to get to the spples.
Yes – squirrels are truly the Hulk Hogans of the local wildlife scene. We’ve seen them carrying huge grapefruit-size Osage Oranges up the sides of trees & balancing them on branches while they snack. Those things are pretty darn hefty.
Ours get chewed by rats, but rats do not take the whole fruit away. Coons probably would have broken branches.
Thankfully, there are no local rats that I’ve seen.
I do not see rats either, but I see their damage. You are fortunate to lack them.
I’ve seen groundhogs climb apple trees and run off with the entire fruit in their mouths. The critters climb very well and run off to stash their prize before returning for another.
A groundhog lived under this shed for a few years, but it’s moved on. I’m sure there are more around, but I think a large critter would have broken these branches.
Groundhogs are definite little Tarzans. Who woulda thought? Last Fall we watched one climb to the top of a large Osage Orange tree & sit there eating one of the fruit.