Just read all four pages of this topic and I can't believe no one mentioned the obvious answer. Sawyer, Jack, Kate & Sayid have all been in the custody of the others AND were rendered unconscious via chemicals. Gee, how on earth did the others learn all this private, not-in-the-public-record information? Sorry for the sarcasm, but I think this one has been evident since Juliet waved Jack's "dossier" at him right after telling him he'd been under the influence of psychotropic drugs.
Ooooh, Dang! I forgot about that!!