Our cat Amelia recently got her teeth and gums thoroughly cleaned at the vet (she was developing some gingivitis), and as the procedure is treated as surgical, she was put under general anesthesia (much easier and less traumatic for everyone involved: vet, vet techs, and cat). Turns out the anesthesia left over in her bloodstream changed her scent for a while, so when Amelia came home that afternoon she smelled completely different to her sister Martha, who responded with hisses and deep growls for the rest of the day. The vet tech had warned me about this before I took Amelia home.
And that’s how I learned about feline non-recognition aggression.
Fortunately both cats are fairly non-violent so there were no physical brawls, but Martha kept her distance from Amelia, and anytime she got close she’d give a sniff, then suddenly hiss loudly and dash away from what she perceived as a strange cat who looked like her sister but smelled like a veterinary procedure. Amelia was confused about this and seemed a bit despondent.
The aggression can last from 24 hours to weeks. In our case Martha’s hissing stopped about 3 days later and she is friendly to Amelia again.
(We didn’t even need to resort to locking the aggressive cat in a separate room or dabbing vanilla extract under their noses as some sites suggest, though we did keep their food bowls apart for those 3 days.)