Sylar (the greatest Heroes character ever!) doesn't eat brains. He uses his "intuitive aptitude" to analyse his victims brains in order to "see" how their ability works. He then copys the ability and leaves the corpse with no evidence leading to him. One tough job for the Police to handle. Claire Bennet is the only one of Sylar's victims to have survived this process. Most Heroes characters know of Sylar and his murders, no secret really.