As foolish as it was, the Atlanta survivors were hoping for rescue if any military / government agencies were still functioning and fighting back. To slightly legitimize that line of thinking, Rick did, after all, see a helicopter flying over Atlanta when he first arrived. Granted, it may have been an hallucination but we'd had no prior indication to suggest that Rick was starting to hallucinate. As far as ransacking stores goes, what do you think Glenn and the others were doing when Rick joined up? Of course they were looking for supplies. But after Rick's arrival, other situations started taking precedence: returning for Merle, returning for the guns, the attack on the camp. They haven't had time to go back into Atlanta a second time since Rick joined the group, and they have deemed it too dangerous to do so anyways. You mentioned Rick's hat in your post in the TVM thread. He's only "found" it once and that was thanks to Glenn grabbing it along with the bag of guns. Not sure about gunshots, though, and why zombies find that sound particularly interesting as opposed to other sounds.