Is this rude if you were a guest?

I reckon they’re going through something or arguing. Bizarre one would disappear then later followed by the other.

I’d be heading home & avoid awkwardness

I would probably find somewhere else to stay. I'd feel uncomfortable and unwelcome and certainly not want to stay for several nights.

Have some people got nothing better to do than go trawling through a poster's history looking for a gotcha moment.

MN attracts the sort.

The definition of formal is "in accordance with convention or etiquette". How is saying good night not formal?
Also note me using "more formal" which indicates a sliding scale between not formal at all (going off without saying anything) and doing a ceremonial "good night" dance in black tie. "I'm off to bed" would sit somewhere right of saying nothing at all which would make it more formal.

Very rude of them! As others have asked, how well do you know them/what's your relationship with them?
I'd have to say something. Just a jokey, 'I didn't realise you'd gone up to bed last night. Hope I was being a bore!' See what they say.
Also, were you all sitting chatting, or watching TV or something?

Who are these people? Friends/family? It's weird whoever they are but just wondering if it's normal in their house to just up and go!

On night one of a 3-night stay? Doubt it.

A hint you'd outstayed your welcome maybe?

It's bloody weird and rude. Just say that you're tired and it's time for your bed. Job done. Leaving your guest there wondering what's going on and if you are coming back is thoughtless. I would have to say something in the morning such as I didn't know for a while if you were going to return.

I'd have just gone to bed too and then said in the morning I didn't realise they'd gone up to bed.

It's rude and weird in my opinion

Nothing like the OP's experience but it jogged a memory. Years ago me and my then bf were invited for dinner by a couple we knew quite well. We were all in the living room when the woman went out to the kitchen to check the dinner and he followed shortly afterwards. We sat for a while waiting for them to come back but they never did and the house was completely silent. After a good long while we quietly went home and made ourselves cheese on toast. I was so hungry by then that I was trembling and cut myself trying to slice the cheese. We eventually found out that they'd had an argument in the kitchen, but it was all incredibly awkward.

It's what they do when they don't have any guests either, I'm sure.

Of course, but they do have a guest, who was left feeling very uncomfortable and wondering if they’d caused offence. Maybe they fart like troopers when they don’t have a guest, but presumably they amend their behaviour when they do have one.

IMHO it’s not formal to say ‘I need to go to bed, help yourself to anything you need, goodnight, sleep well’. It’s basic communication, which helps to put other people at ease, as they know what’s going on. Me and my adult/teen kids say goodnight to each other and we live together.

It was. The more persistent posters need to read the talk guidelines.

It’s so annoying when people go digging around previous posts trying to go trip others up.

I’ve had this happen op, friend invited me to stay for a night - seemed v enthusiastic. She showed me to my room and said wait here while I have a quick wash, I waited ages, like 1.5 hours and in the end was so thirsty I went downstairs to get a glass of water. Found her down there watching tele! Felt quite awkward but said mind if I join you, we watched together for a bit then she disappeared and after a long time I realised she’d gone to bed. I was so baffled and uncomfortable so decided to head off straight after breakfast. In the morning I went downstairs to find she’d finished her breakfast, she said oh I’ll just pop upstairs and was gone an hour! After which I said right I’ll be off now and she seem absolutely stunned and disappointed that I was leaving!
it was all so odd. She’s since been diagnosed with adhd, don’t know if that has anything to do with it but it was the most peculiar visit.

Maybe they kept dropping hints as it was late and OP didn't realise so they gave up and went to bed .

Some people have not only trawled though the OP's previous threads but proudly announced that they've reported. If you're unsure you can simply report and let MNHQ do their job.

I think you post needs more detail . You arrived
at what time , did you have dinner together . See your room. How well do you know these peopel and who went to bed first .
Also were you on your phone as asked , were they yawning . Did they say they were shattered ? Do you know them to be early bed people?

How is saying goodnight being formal?

My guess is that this is a complete mis-communication. If it’s a couple that you are staying with then they both think that the other one has said “we’re off to bed now” or something like that. Or could one of them have said it quietly or out of ear-shot thinking you could hear?

Are they normally strange with anything else?

I’d lightheartedly mention it to them in the morning “oh I was sat downstairs and realised you’d both buggered off to bed last night!” - then see what they say. Don’t over-think it or let it spoil your visit.

You were not being unreasonable, and the troll hunting was ridiculous.

Definitely rude and weird

I would find it very strange and also wonder if I had done/said something they took offence at.

Yes, strange and rude!