This poses somewhat of a problem: it has been established that the walkers sense the living, and follow them until contact is made. After Hershel's group had been exploring the woods finding walkers, whatever movement/odor/energy sense used by the walkers should have set season two's conclusion in motion long before the time period in that episode, or at least a version of the invasion.
It appears walker movement/detection is changed to fit the story, without realizing the audience does not gloss over details, contradictions, etc.
Has it been established? As I understood it, all we really know is:
1. Walkers have a rudimentary hive mind
2. Walkers tend to congregate over time.
3. They are attracted to noise
4. If close enough to food they can smell it.
5. Their sense of smell, however, is pretty close range and covering someone in zombie guts or having a 'tame' zombie nearby can fool it