User blog comment:SayuriDarling/The Mark of Athena Official Discussion/@comment-4388405-20121006181920/@comment-3323804-20121006213258

Also I don't see a reason why an Eidolon would want to hang around for six months spending quality time with Piper. Also, if an Eidolon did posses Jason since The Lost Hero, then it probably would've tried to stir up trouble in Camp Half-Blood or start the civil war between the two demigod camps using Jason. And I don't think the Eidolon could even enter Camp Half-Blood due to the boundaries.