Or it is just part of Ben's manipulation. Be buddy-buddy with Jack and then send him back to his peeps. Naturally the rest of them will be suspicious, no matter what Jack says or does. He asked Claire all sorts of questions when she was still pregnant too, those are just standard check up questions. I think he is acting the same as he did before he was taken, except with Kate. Hanging around Juliet is the main thing that is worrying everyone but think about this, if he came back from the Other's camp and had something to hide wouldn't he distance himself from Juliet? To keep the losties faith and trust in him. He would not be sticking up for her and sharing oatmeal with her IMO.