When you have mulitple cats and have to take one of them to the vet, often when you return home the other cat(s) hiss at it and act like they've never seen it before. Cats rely on scent SO much that when a cat family member makes a trip outside the home and returns with new smells/not smelling like the home environment, they are seen as outsiders. It's much better for the cats if you put the absent cat in a room my itself and do some scent swapping before you let them have contact.
I see this too in the shelter; two cats are surrendered together (often even in the same carrier), go through the vet office and put in separate kennels, and when I try to reunite them in the adoption room, just an hour or so later, they act like they've never seen each other before.
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