Have the Others said anything about the castaways that a normal person couldn't find out through intensive research? It wouldn't take that much paperwork to figure out Sawyer's real name and Sayid's past and I bet the police figured out Sawyer shot that guy after the plane had taken off--he shows up, makes trouble and gets drunk, is seen near the place, maybe there's even documentation of him owning the gun, he leaves right after... pretty suspicious.
Ben must think they're so dumb. First he snuck in a couple of Others and it took them weeks to realize it. Then he wandered into their camp himself and even with Rosseau yelling about him being an Other, they still let him free. Then he sends in an Other who they KNOW is an Other and yet they STILL let her walk around. They're morons (although I guess they couldn't have known about Claire having an implant), especially Jack. "Gee, I know you want to get home more than anything, so that makes you one of us! Even though the only person who has a chance of getting you there is Ben."