Continuity mistake: When Glenn wakes up in the prison you can see two chairs leaning against the walls behind him when he stands up. These are still present in the second shot, but when the camera moves back for the third shot they have mysteriously disappeared.
Continuity mistake: When Glenn is with Tara and getting ready to move out, he puts on his helmet and you can see it is strapped on. When the shot changes, he swings the fence open and you can see his strap is lose. The camera angle changes again and the strap is back on again. (00:38:15)
Character mistake: At the very end of the episode Tara tells Glenn that her sister and nephew were not supposed to be there. Tara had a niece not a nephew.
Continuity mistake: When Glenn wakes up we see the door behind him is flush with the concrete, but when he goes inside you see stairs he had to descend to get to the door. There is also no scene in the entire prison episodes where an outer orange door is preceded by steps, they are all flush. Also If you look at the door behind him you see two folding chairs leaning up against the door. When he turns to open the door the chairs are gone.
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.