I don't think that is how roaming works. Although I have no immediate proof, as far as I know, roaming Pokémon don't hide in a specific grass patch, they "hide" in a route where they have a chance to be encountered and can be encountered while repel-like effects are in place. So if they were hiding in the daycare route, any properly coded grass patch may encounter them, but you don't have to find a specific patch, as that would make roaming Pokémon nearly impossible to find. As for before the roaming Pokémon are released, they are a piece of code that hasn't been executed, and therefore only exist in the game's files, in a sense, somewhere inaccessible, but not in the conventional "in game" sense. :P