Non Recognition Aggression

So sorry if there's already a thread dedicated to this. If so, can you direct me to it?

Two female cats - best of friends. One went to the vet due to vomiting, ended up having an obstruction and needed surgery, meds, etc. Finally was able to bring her home after 3 days at the vet hospital. House kitty is now hissing, growling, and stalking her. Thankfully no actual attacks....yet. Vet kitty only growls back, but seems very sad when she walks up to house kitty and is rejected

Last night I kept them all in separate rooms but I don't want to keep any of them cooped up. Vet kitty has been grooming herself non stop. Any tips on what to help these girls get back to normal? I read stuff about vanilla extract or a diffuser? Does anyone have experience with this? I feel so bad for vet kitty

Perhaps house kitty senses vet kitty's vulnerability now and is confused by it. Worried that she may get sick too. Also, vet kitty does not smell the same now as she did before she left. I'm sure it will all resolve itself.

Hi there, there's some threads on what you are asking about and I found one that's quite relevant and short to the point. Give it a read and see if suits you.

http://www.thecatsite.com/t/32783/vanilla-extract-on-cats

This is fairly common.  Vet kitty smells like the vet and sickness; not herself.  I have this issue between my girls any time one or both have gone to the vet.  Cats know each other by smell and just 20 minutes in the vet's office is enough to make a litter mate smell like a total stranger.  Noodles is always the one who does the hissing.  It usually takes about 3 days to get back to normal but its taken as long as a week and then sometimes she never gets hissy at all.  I did find Noodles hates the vanilla; so I haven't done that in a long time.  I try to pet both a lot and talk to them calmly.  They have never taken it beyond the hissing and some stalking from a distance.  If it gets physical I would separate or do something to distract them; such as clapping my hands or making some noise.  Some of the techniques for introducing cats can help; such as scent exchanging and feeding meals progressively closer together as they tolerate it.  In some cases; people do have to separate and reintroduce their cats as if they have never met.  But I would say only when they are getting into actual fights.  I think supervising them for now is a good way to handle it.  Especially since one is recovering from surgery.

This happened to me a few days ago if you want to look at the thread I posted!! In our case it resolved after the cats were both home for a few hours and spent the night apart. I found that rubbing a blanket on the non-aggressive cat helped the aggressive cat recognize her. Then we fed them on either side of the door and opened it a few inches to judge their reactions, and it was like nothing ever happened. Good luck! I know it's hard to see your cats like this