How did Michonne find the prison? It was not like Glenn and Maggie drove in on a dirt track.
Perhaps she's so familiar with the general area, that it was a logical assumption of where two cleaned up people would be hiding out. They arrived in town by vehicle, so she probably thought they are not hiding anywhere nearby, neighborhoods are zombie no-man's lands, so where else would they live?
I love how everyone looks all sweat stained and hardened and then there is that cute, fresh, pink-clad baby.
I think they would go out of their way to keep the baby clean, even if their own habits fall by the wayside from time to time.