Why does my cat act hostile towards my other cat after being kenneled for a week?

Question

I have three cats, a male red tabby (Ed) and two black cats, father (Boo) and daughter. All three cats have lived together for the majority of their lives (13-ish years). They get along fine, aside from the normal morning batting at the food bowl between the males. They've been around each other and coexist quite well.

I just got back from a week-long vacation. We had to kennel all three of our cats at the vet. There were no issues reported.

Upon releasing all three cats back at our house, my red-tabby - Ed - started hissing at the black male - Boo. He then ran away and promptly tried to spray on a box. Both Ed and Boo have sprayed before, but it's a habit we have broken. Ed went on to attempt to spray on several other surfaces.

When separate from Boo, Ed acts perfectly normal. He tries to sniff everything - totally expected - but other than that he is responsive to me and purrs just fine. When he sees a black cat though - Boo or his daughter - he tries to get away and spray on something. If he's cornered he'll growl or hiss at them as he rushes by.

Why is this, and what can I do about it?

Keeping the cats separate is a temporary option, but it doesn't work too well overnight, as the cats will invariably want to roam around the rest of the house at night, and complain loudly if they can't. Not very conducive to sleeping. Is there perhaps a way I can convince Ed that he doesn't need to defend his territory from Boo (I assume that's what's going on)?

Interestingly, neither Boo nor his daughter have the same behavior. They are simply sniffing everything and slowly returning to normal. They exhibit no other strange behavior, and act fine around Ed.

EDIT: As of the morning after, Ed is now meowing incessantly. All night long he meowed non-stop, waited 10-20 minutes, started up again, stopped again - all night. He started doing this soon after we brought him back and he got his bearings.

The meowing has lessened somewhat this morning, though he is still doing it frequently. He is sniffing Boo's daughter and seems fine with her. Boo has been separated all night.

Answer 1

If at all possible, I put my cats into shared 'cabins' in the cattery. From experience, I know this means they don't lose the scent of one another on themselves, which is important to them in identifying who is part of their 'group' and who isn't: Do your cats have a group scent and How cats create a group scent

I learnt this when one of my cats (who had always lived with his brother) went missing for two weeks and then had to stay at the vets for a week. Upon his return home, neither of the two cats recognised each other until they had been together for a few days; until then they fought like cat and cat.

I accelerated the re-introduction process by rubbing the bedding that had always stayed in the house (and been used by the cat who didn't get lost) over the prodigal cat to remove the vet smell and add a brother smell to the mix; however, if nothing else is going on apart from being separated for a while, things should settle down with time.

The bedding technique also works if your vet will let you send bedding with the cat -- it helps the cat settle because the surroundings smell familiar, and also helps them to retain the 'group scent' for when they came home.

It may be that the father and daughter naturally smell somewhat similar so have not 'lost touch' with each other's identity, but it will take Ed a while to reintegrate.