User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-10901795-20130815072908/@comment-3146930-20130815103749

It is a good idea, but I don't think it will happen. Even if Percy does retire after beating Gaea, he is still a demigod (and a powerful one at that) and monsters will always be drawn to him, whether he likes it or not. As far as Riptide not returning, the time it takes to return to his pocket ranges from a few seconds to a few minutes. It is possible that because Percy was so far away, the sword may have taken longer to appear in his pocket.

I also am not sure if Riptide ever actually made it back to Poseidon. Chiron explains that Riptide has had a tragic history, hinting it had other owners besides Hercules and Zoe. Chiron also says that it is a gift from Poseidon, but he has kept it for years. Maybe it never made it back to the sea at all. The line return to the sea for me, I always thought when Percy died, they would send him out viking style by pushing him onto a burning raft out to sea.