User blog comment:Frodo the 9 fingered/A Third Camp?/@comment-5112532-20120731101113/@comment-3146930-20120809032846

Annabeth lived in San Francisco for almost a year and didn't even know there was another camp of demigods. Jason also mentions that they could look forever and never find it. The Mist is a powerful thing, preventing anyone from finding something not meant to be found. Unless you had a guide, each camp wouldn't be able to find the other as the gods keep them apart. If there is a third camp, then the two others most likely wouldn't be able to find it unless they had a guide of their own.