The Mark of Athena question

I think I may have started that rumor. I remember clearly that Percy freed her from Ogygia, it's in one of the last chapters in The Last Olympian. Also, I follow Rick Riordan on Twitter and he does Q&A's from time to time. Someone asked him about Calypso, and he replied that she would definitely make an appearance in the Heroes of Olympus series.

It's not a rumour, she is coming back, Rick Riordan said so in his Q&A thing...

I really want to see her again! I liked her character and I hope she gets to see Percy again... It might make Annabeth a bit jealous, but who knows, they might still be in Tartarus when she appears and won't even get to see Percy again...

I am very curious about this one!
The gods promised at the end of The Last Olympian to free her (i think)
hope we see her again

A perfect ending for the series would be Percy and Annabeth on her island for eternity. By that time they will already have saved the world from two major powers, and will be deserving of a peaceful ending.

She appears in HoH and BoO.