User blog comment:SayuriDarling/The Crown of Ptolemy Official Discussion/@comment-24055022-20150419005650/@comment-25044056-20150419014846

I'm glad that Leo hasn't shown up or been mentioned in any of these crossovers. It fits into my theory of what happens after the Blood of Olympus.

spoiler warning for Blood of Olympus

The final line of the Blood of Olympus is "the bronze dragon spread his wings and they soared into the unknown", my belief is that Leo and Calypso traveled the world together instead of Leo returning to camp half-blood. I also think Leo may have remained dead for a long time before his eventual resurrection.