As the crowd gathers, heading off to find a good place to build a barricade (because that’s what you do if you’re in Paris and you want to revolt), they start picking up more and more people. Pere Mabeuf joins in, though he seems to have suffered a BSOD. Gavroche is having such a great time that he doesn’t notice a disguised Javert. Courfeyrac stops by his apartment to pick up his hat, where he runs into some boy asking for Marius who looks like a girl dressed as a boy. It doesn’t occur to him that she is a girl dressed as a boy.
In fact, it doesn’t occur to anyone that Éponine is a girl, because she’s wearing pants.
Her raspy voice probably helps the disguise. But Marius almost recognized her by that voice, and dismissed the possibility.
I’m reminded of the scene in Mystery Men where one of the D-list heroes is explaining that the Clark Kent stand-in can’t possibly be Captain Amazing, because he wears glasses. How could he fly with such bad eyesight? Also Sandman: World’s End, in which a girl dresses as a boy so she can be a sailor. Only one character sees through the disguise, remarking that it’s about seeing what you’re actually seeing, not what you’re expecting to see.
It’s a blind spot. But it’s also refreshing that everyone goes with the gender she’s presenting as (even though it’s a disguise for her, not an identity) instead of trying to police it.