To be frank, even if it was on DI since the end of Kh1-the ending third of kh3 (which it wasn't), the explanation people are using for how it got there (Aqua not picking it back up in the end of 0.2) is so contrived and full of far too many holes to come off as anything more than trying to justify bad writing that the writers themselves didn't even care enough about to explain.
You mean tell me that no one saw it lying there once for the entirety of the series? Roxas and Xion (both of whom collected several sea shells from that beach on two separate occasions) just so happened to completely miss it on their missions there? Riku, who was present there with Xion, and Kairi, whom Roxas saw on the island, also didn't see it (despite them knowing that beach since they were children)? What of the reunion at the end of KH2 where Sora, Riku, Mickey (who's actually seen the Keyblade before), Donald, and Goofy were all present there at once? Did they not see it then either? Cause I would imagine, what with how excited he was to be home, Sora would have had a feild day on that island. Or perhaps the post credit scene of KH2 where Sora, Riku, and Kairi are just chilling out there after Recoded? How is it that no one, anywhere saw that big metal key lying in the bright white sand?
And if it just happened to wash up sometime during Kh3, my my how convenient that it took well over a year to finally show itself. Aqua's later means of defense opens a whole nother can of worms, but I won't go into that right now.