The young girl, no older than nine had been hiding from the 'walkers' (as she called them) after her caretaker's recent death, and the absence of the last remaining members of her probably former group. She had found this store, and it seemed safe; safer than the other parts of the city, that is. Though she heard the footsteps of someone coming in before the music played, she had chosen to stay put- maybe they wouldn't notice her.
Though the sound of the music made Clementine think that walkers might come- So she had initially gone to see if the person left, so she could turn off the music, see if there were walkers near or not, and if not, leave. Though, she noticed a figure- clearly human, as she walked over to the spot with caution- she was getting scared, and so she slowly crept from behind a wall- sticking her head out a little too in the clear, to get a better look at the person, who was pointing the pistol in the direction she was coming from.