We had the exact same thing happen with our two cats. One went to the vet for a few hours and when we got back the cat who had stayed home had completely forgotten the cat that left. He carried on all day, that night and the next day. Eventually he stopped with the hissing and puffing up, but it was more like he was accepting this new 'impostor' cat, rather than realising it was his brother who he grew up with. LOL!
I think it was mainly because they cat that went away came back smelling different and acting a bit funny (from the anaesthetic).
Anyway, they are totally fine now and back to playing and cuddling up together. I hope yours stops the carry on soon