Humans are actually surprisingly intelligent and adaptable. With patience, they can be taught a wide variety of quite complex behaviors.
I found that the kibble bowl has dried saliva on it, so I’m sure the old kibble is covered with saliva too. But I have been one to stir the old kibble to make it look new again.
But, like campp’s cat, can they tell the difference if you put the kibble back in the bag and then repour it? If you pass it off as new, do they care?
Thanks for the picture. Mitzy looks like a classy kitty. At least someone complies with the written rule about cat pictures. 
No cats for me but my (late) bulldog had a peculiar eating habit. She didn’t like to dine alone. I would give her a scoop of kibble in her bowl in the kitchen. If I was in another room watching TV or on computer, she would grab a mouthful of food and then drop it about 10 ft from me. If I left her alone, she would eat every morsel. But if I yelled she would run away and never eat the kibble on the floor/carpet so I would have to clean it up.
I learned to let her eat the way she wants to eat. She had no problem with leftover kibble.
This is Jack. He has to be petted before he will deign to eat. And this is the ONLY time he will let you pet him. Try any other time and you’ll pull back a stump. But while being petted, he insists on rubbing his head against his water bowl and getting water on the tips of his ears.
He also will not eat kibble that’s been left in the bowl. Gotta be the fresh stuff.
I dunno- cats are crazy, no doubt. But doubtless they can tell the difference, as they have a pretty good sense of smell, and cat food that’s been sitting out for a day wont smell the same.