I went round in the evening again to the three spots. The food was eaten in two spots. I'd like to think perhaps Joanie had come to eat.
There are two cats who look very much like Joanie in the alley. They are also grey tabbies with white paws, but on closer scrutiny, it's not her.
I ended up feeding some stray cats.
My daughter, who is taking a course in vet pharmacy, asked her lecturer about Joanie's case, and the lecturer said Joanie is probably feeling much better - that is why she left. He also said it is probable her infection has cleared and she does not need antibiotics anymore.
That's comforting to hear.