You are doing the right thing, keep the physically separated but so that they can smell each other. Try to feed them at the same time so that they can smell and later see each other while eating to associate good things with each other. But keep them physically separate for a few days.

I haven't had cats in years but I asked my vet friend who directed me here; https://www.aspca.org/pet-care/cat-care/common-cat-behavior-issues/aggression-between-cats-your-household

It has some specific advice on cats who are familiar with each other who start to fight. I have heard the tuna juice trick before.

It might be a smell thing. My friend has 2 cats, litter-mates, and they would do the same damn thing every time one came back from the vet. It would settle down after a few days when the smell dissipated. She used to have to take them both to the vet even if only one needed to go. It's called non-recognition aggression.

Then the vet changed their brand of disinfectant and it never happened again so they never got to try the following advice the vet gave them:

What you can try is de-outside-smelling your escapee by giving them a damp-cloth bath and then rubbing them all over with your worn clothes to reapply your smell.

Also, maybe stinking up your aggressive little idiot by leaving the same clothes with outside smells around them. Pick the clothes your aggressive cat loves to snuggle the most if they're that type of cat. If you can mix up their smells enough, it will hopefully break the association of "smell = the other cat" and your aggressive one will begin to recognise the escape artist again.

Give it a few days. It's the smell. My guys do this if I give them a bath. They also often do it after they've been outside.

Are you sure it's Noki that came back and not another similar looking cat 😂 (j/k)

I really love all the great advice you're getting, and I hope it works, but now all I'm thinking about is if people had the same problem and couldn't recognize each other when their smell changed. 😂

Had an incident where I put on cologne and my cat COULD NOT recognize me. Hissing, feral growling. I think it was some kind of uncanny valley thing where I looked the same but didn't smell "right".

Could be he just picked up a weird smell.

Sounds likely to be a scent issue - Noki may have gotten the scent of another cat or animal on him which is confusing Scooter.

The steps you're taking are good and might take time, I'd stay the course as if they are strangers and take the introduction slowly. In the meantime, there's a product called Feliway (might be a different name where you are) that is a pheromone blocker, it can really help mute the stressful effects these scents can have. They use it in vet rooms a lot. Comes in a spray or diffuser, might help you.

Good luck, I hope it resolves quickly!

I had a cat who was best friends with a neighbour's black cat. We went on holiday and another black cat attacked my cat there. When we came back, he kept hissing at his bestie for about a week before realizing his mistake. So sometimes it's not the smell, sometimes cats are just a little bit dumb.

Edit: I should add my cat was orange.

This happened to us a couple of times when one cat went to the vet and came home smelling wrong. The only way we could get the other cat to "reset" was to give the wrong-smelling one a bath.

For my cats it's plain old jealousy. One is jealous the other got out as they are both Indoor cats. If it isn't fighting its urinating on the escapees bed to prove a point. They are middle aged men now but still so petty. I let them fight it out and then it's done.

hey did scooter start recognising noki?