Arrow on the Doorpost - S3-E13
Continuity mistake: In the final scene of the episode, Rick and Hershel are talking while looking out over the prison yard. Near the end of the conversation, Rick squarely faces Hershel and says "I'm hoping you could talk me out of it." The camera then changes to Hershel, and Rick is now immediately facing 90° to his left looking over the yard. When the camera changes back to Rick, he is now looking directly at Hershel.
Answer: Walkers are attracted to noise, they can tell who's dead and who isn't dead, in S1 when Rick and Glenn cover themselves in walkers' guts they're able to get far by walking through the herd of walkers because they smell like they're dead, but as soon as the guts get washed off by the rain the walkers immediately attack them, if you draw attention to yourself they will attack you which we've seen that many times throughout the series that it's not always successful. Whether you're covered in guts hiding under dead corpses or under a car.