If you change your location by foot (like enter a new route or building) the Poke will, at random, travel to an adjacent route. If you check your Pokedex for the location of the Poke, it will tell you where it is. The best way to track a roaming legendary (like Mew in CC and the dogs) after you have 'seen' it in your Pokedex is to put yourself at a major junction route with some grass in it. Speaking from my experience, since there seems to be a shortage of info out there on doing this without losing your mind.
