User blog comment:SayuriDarling/The Mark of Athena Official Discussion/@comment-4388405-20121006181920/@comment-4485028-20121007033841

I'm going to have to side with Dark on this one. I don't really know for sure when Jason was possessed, but the eidolon wouldn't have been able to make it inside Jason's body due to Hera's divine form. Also, if he had tried to return to CHB he wouldn't have been able to due to having a monster inside of him.

Although a counter arguement could be that Hera just teleporting them into camp (Piper's foot on Drew's pizza) completely bypassed the magical borders.