I've also had this problem but it was usually only with my RB Storm. He would not only hiss and growl at a cat that had been at the vets but if he was the one returning home from the vets then he would also growl and hiss at any cat that wanted to come up and sniff him. I would then try to keep him back in my bedroom and usually by the next day everthing would be fine. Good luck.

The groomers also has the same reaction. When a few of my cats had been at the groomers for their lion or kennel cuts, then the other cats would also think that they smelled funny. Alani didn't recognize my RB Storm one time and he was trying to stalk him like he was a wild animal. Luckily after a few days this behavior stopped.