My auntie and her family went away on 16th July. I always look after her cats everytime they are away. I go day and night, feed them and stay with them a few hrs. I went Sunday night around 8.15pm and cats were fine. (She has two cats, male and female. Brother and sister) Both were happy, looked very healthy and otherwise their usual self. I go Monday morning and as soon as I entered the house, I knew something was not right. Usually when I enter both cats come running down stairs, that day though.....nothing. I switched the alarm off and called out to them. Got no response. Then I looked up the stairs to see Millie (female) peering her head out of the banisters. She looked like she had just been frighten. She slowly walked down and at first I thought she had hurt herself. So I went up to help her and that's when I saw....Tommy (male cat) lying on the landing. I knew instantly that he was gone. Millie slowly crept passed him and went down. I couldn't see any external injuries...besides during the nights I leave them in and during the day they are out but due to weather here they had both been in since Saturday evening. They popped out for about 15mins Sunday morning and Tommy popped over the garden but was back within 5mins. When I went back in the evening they popped out again for a mere few moments. So essentially they have been in since Saturday evening. So we can rule out RTA or anything like that. Tommy was fine Sunday night and Monday morning... dead. I am struggling to understand what has happened. I brought him to the vets my aunties use and he will be kept in cold storage til they are back which is Friday morning. Vet had a check and could see nothing externally to explain what had happened. So it's obviously internal. He was around 8/9yrs old and very very healthy. In the space of 12hrs he went for being alive and great to suddenly dead. I'm just in utter shock, this was not expected. Tommy has been the perfect picture of health since birth, in his many yrs of being alive, never was he at the vet other then for a usual annual vaccination or for a groom. He was a big boy and very very long haired.

What can cause sudden death in cats?

I would guess it may be a sudden death for much the same reason as a human - heart attack, stroke, anurism.
Poor puss - I feel rather sorry for his sister, but at least she knows he's dead and wont be fretting over where he is.

Millie....is doing OK. She seems to be coping well, only thing I noticed was she was more clingy.

I hope Tommy didn't suffer.... It was all so sudden. I suspected Heart Attack to, just wondering what led to it. He was in tip top shape. But....I did find out that before they went away he had an annual vaccination but he was panting but vet put it down to nerves cause he hated trips to vets.

My own guess would be sudden massive stroke. But obviously, the vet will tell you.

At least he died without months of decline and sickness.

My 3 year old Siamese died last year of cardiomyopathy (clot to heart). He went from fine to dead within 10 minutes. It was in the middle of the night in my bedroom and was heartbreaking to watch.

A friend lost his pet dog once, it died of carbon monoxide poisoning from a faulty boiler.
Was the cat sleeping near a gas boiler, or other gas appliance? Do you have a CO alarm? It could be worth checking.

Autopsy at the Vet's. No other way to be certain.

I would go for a necropsy tbh, could be a poison or trauma, most likely just undiagnosed heart issue, or a stroke, as said.

Wow, well it really can be anything in my experience. Accidentally eaten some poison, suffered some trauma, random heart attack. but yes as mentioned in previously in the thread, you can ask a vet for an autopsy to be certain should you wish.

My cat died randomly from a seizure a few years ago, up until that point I would've said she was really healthy... she was old though.

Would want to go through autopsy/necropsy especially if you have other pets, or intend to get another pet just to make sure it was not a "man made" cause so to speak as another poster suggested

Cats survival instincts can make detection pretty tricky unless you are really in-tune with your pets routines and typical behavior and even then unless its visible they really can hide things well

Sorry for your loss