Personally I have not read the books but, if she were able to have seen it, she wouldn't need to wait for it to be at a mark or certain point (Unless it being where she knows it's head would be). It could of course have been with Abes help but going off without his help..
In the movie she has outlined where the Hollow's corpse will be each time, if you pay attention to the scene she watches the grass and stays silent in order to hear it. Emma has to tell Jake to keep it down in order not to throw her off, him speaking or making a noise would've definitely ruined her set up.
She could've misheard or the Hollow could've simply gone over to them instead, thus creating a bit of a bigger issue.
Looking back at the grass, it not only makes noise but it is tall grass and a patch of it in a specific area. That area being where she found out where the Hollow had come from. It also happened to have come from the ocean, meaning there'd be water there as well to help indicate it's position. I cannot say how she would've known where it's head would be and not to have assumed it'd slouch but that's my view of it.