It can't be true Thomas doesn't know when, or rather why this happened. Maybe he was too stressed, maybe he was too naive, maybe this was too well done. Thomas likes to think that everyone else was as fooled as he was. But first, lets recap. °°°°°° A call? A call from Polly? That is indeed a rather unfortunate way to end the day.
"I'm going to be at your house in a bit, it's important"
Thomas was a little scared to hear his aunt requesting his presence with that urgency, since that day had been a day that all the Peaky Blinders were assigned to do... certain tasks, all of them. °°°°° Not an hour later Polly was already there, the house staff had let her in and lead to a room in the residence. Thomas' surprise was that his aunt was there with one of the Peaky Blinders, one of the newest ones, one that Arthur brought along one day, Thomas didn't had much saying in that.
"Next time you have a new Peaky, do an actual research, you got yourself a disguise master."
Of course Thomas doesn't really understand, and he looks at his aunt for some clue, and some seconds later, Polly has some mercy on him, so she gives him a clear clue "When the little task was over, he came back wounded, but when I got to help him, well, it turns out that she is a woman."
Thomas stays frozen for a good minute, is his aunt trying to tell him that one of his men is actually a woman? He can't even humor that thought The next moment is a blur, for him this is funny, also maddening, he understands why a woman would disguise herself as a man, but to fool him and the integrity of his gang is very different. Somehow he is already grabbing the woman's collar, now that he sees her like this, even if she is still in a suit, well, it is sinking in, she is a woman, and it seems that no one else knew about it
"I'm going to give you five minutes to explain yourself."