Secret singing area
In Pokémon Black 2 and White 2, there is a secret room under the bridge where a girl is singing; upon finding her, she forces the player outside. There is a girl who will reward the player for finding the source of mysterious voice.
Bulbapedia entry on the Village Bridge
I assume this is the girl you're talking about? I haven't been to the Village Bridge in a while.
Oh I've found her, she's on the bridge (not in any of the rooms), and is right near the edge just before the houses (if you're going in the direction towards route 11/ Opelucid City).
Marriland's video; get to about 20:40 mins, that's the girl he's talking to.