My cat Stinky always reacts that way towards Tiger or any cat that has been at the vet and has been put under anesthesia. I think they can smell it. I usually stay in the room with them to keep Stinky from harrassing him and then after a few hours she seems to adjust and she re-accepts him. He's been put under a half dozen times this last year so we've had lots of adjusting and re-accepting going on! lol