In The Son of Neptune, Frank says that some half-bloods leave Camp Jupiter to follow Greek ways. However, the Romans do not know about Camp Half-Blood, so the half-bloods that leave may not find it. What if they formed their own camp, thus making their own third camp? If this is the case, (I realize it is unlikely) then the seventh might be from this camp (don't just say Annabeth is the seventh and nothing else, as that is not confirmed). Also, I wonder what the name of the camp would be, my guess is Camp Zeus, just to spite the Romans.