If you've read the first two chapters, tu know the cliff ending that says Percy is missing.
Percy is a powerful demigod. He's a son of the big 3, a great sword fighter and he bears the curse of Achilles. He's defeated a lot of monsters, and made a lot of ennemies. Who kidnapped him?
Some would say the Anemoi thuellai kidnapped him. Maybe they're on a demigod kidnapping frenzy, which would be why they're after Leo and Piper. But thing is, they want to kill them, not capture, and they never mention a collection, o joining other heroes o anything like that.
Maybe it was a god. Ares is well known not to like Percy. But wouldn't they be scared to offend Poseidon?
Staying with the ide aof all powerful imortals, what about a titan? Could Kronos ahve reformed and tooken over another body fast enough? I don't think so, according to Hermes in book 5, he is spread thinn, for now. Another titan maybe?
Either way,Percy is kidnapped, where is he?
Underwater is one, very faible, possibility. Due to the lack of sons and daughters of Poseidon, not many would be able to find him underwater, yet Percy would still be alive and kept prisoner. But being underwater automatically strenghtens Percy, so he'd be strong enough to escape. Besides, Poseidon would be able to locate him, and his opinion of what is need-to-know for demigods is pretty good, so he'd tell somebody.
Back at Ogyjia perhaps? No. No man finds Ogyjia twice and he would probably have come back.
That leaves two places were Percy isn't in his bets strenght. Somewhere aerial and the underworld. Again, the problem with this is that Zeus and Hades would easily locate them. Plus none of the two are very fond of Percy, so they might kill him ASAP. The abducter might not want that.
There are probably other, and better, theories. If tu have any, I'd like to know.


ps this is my first article, please be forgiving