Factual error: In the scene at the fire truck, Abraham uses the deck gun to kill walkers. However fire truck water pumps are powered by PTO from the engine, which isn't running.
Continuity mistake: Maggie pulls the ladder from the rear of the firetruck, but in previous shots the only ladder on the firetruck is hung on the passenger side.
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.