My extensive headcanon on why every single person in Downton Abbey seems to know that Thomas is gay
Of course if we were to ask this of Fellowes, everyone probably "just knows", but I am not satisfied with this answer. So. I'm now going to go through every person who somehow knows about Thomas seemingly without a reason, and give my headcanon explanation.
Miss O'Brien was the only person to find out by Thomas willingly "coming out" to them. It was an early point in their alliance and Thomas probably needed her help with covering up a mistake that could end up with him getting caught. O'Brien didn't care and helped Thomas scheme his way out of the situation and that cemented Thomas's trust in O'Brien, along with the thought that this would never be something she could use against him.
Mrs Hughes noticed when Thomas formed a small crush on someone in the Abbey. Like another male servant. Let's say for headcanon's sake that it was Mr Watson, his lordship's former valet. Mrs Hughes noticed Thomas perking up whenever Watson paid him attention and being much more willing to help him when asked than he ever was with others. Mrs Patmore noticed this as well and mentioned it, to which Mrs Hughes replied something along the lines of "well, you know lads when they're soft on someone".
Mr Carson only "suspected" Thomas before the whole Jimmy thing happened, and the reason he suspected him was that he overheard some joking between Hughes and Patmore, but he chose to ignore it. It still affected the way he saw Thomas.
Mr Bates found out during the Duke's visit. He first saw how the Duke went through Thomas's room, then heard Thomas returning to his room late at night, sounding distraught even through walls. So Bates put two and two together.
Bates later told Anna, when she tried to find out why Bates was so angry about Thomas taking Daisy to the fair. "You never know, Mr Bates. Maybe he does really like her and is just not great at showing it." "I don't think Thomas is capable of liking Daisy." "That seems a little harsh. Everyone is capable of love." "Oh I am sure Thomas is capable of love. Just not this kind of love."
Bates also told Robert through a series of joking allusions like the one in canon, you know, "I think he cheered up when he saw the gentleman". Bates knew Robert would be fine with it and probably amused so he doesn't see why such things would be wrong to say.
Anna later accidentally told Mary and felt pretty bad about it. Maybe Mary said something like "now that Sybil's left with a chauffeur, we must be careful that Edith doesn't elope with Thomas or something" and Anna responded with "I do not believe that will be a problem, my lady". And then Mary grilled her until she told her the meaning behind her words.
Miss Baxter knew Thomas already as a child and once when he was a teenager she caught him with someone (and told his parents because she was concerned).
The random butler Mr Moore with a superhuman gaydar who called Thomas "a delicate-looking fellow" is one of those nosy guys who in the modern day would read through someone's social media before interviewing them for a job. He knows lots of people and when he asked around about the men he was going to interview for the assistant butler position, one of his friends said something like "you'll want to be careful before hiring that Barrow, I've heard he has... questionable morals".
Chris Webster saw a good-looking guy, hoped he'd be gay and was delightfully surprised.
