Other mistake: Earlier in the season Father Gabriel Stokes burns his clerical collar, but in the end of this episode he is wearing it again. Especially doesn't make sense, because he said he gave up being a pastor.
Other mistake: When Gabriel is out by himself on the road and kills two walkers, he is wearing a white shirt. Blood is shown spattering everywhere, but his shirt remains spotless. (00:37:00 - 00:39:00)
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.