Neville liked to think that he would have noticed the rat, eventually. The lack of hearing meant everything had to really get into the light before he noticed it. But he was thankful (after the slight terror of it whizzing by his face) for the knife that landed in the beast. The whole thing was a bit more gruesome than he normally liked, especially when only lit by the small glow of his wand, but there were sometimes necessary evils and it was certainly justified in this case.
He stood up again, briefly cautious in case the person who came across him was just killing the rat for their own sake, rather than helping him, but he was slightly relieved when his wand cast its glow on a familiar face. He opened his mouth, trying to speak, but nothing came.