In this picture of Ace's and Whitebeard's graves from chapter 668 cover page left you see the cleaned version which was included with the tankobon format of One Piece, right one is from the jump in which the texts were covering an important part of the picture: the three cups in front of Ace's grave.

These three cups are known from chapter 585 "Cups of Brotherhood", Luffys, Aces and Sabos flashback in which Ace stole some of Dadan's booze and they decide to drink from the three cups of brotherhood to celebrate them being brothers. So basically no one else than these three could even know about the cups, so the only one capable of putting them on Ace's grave is Sabo.

Of course the fans recognized this immediately and one of them even wrote to Oda about the cover page in SBS 68: it's roughly translated like this:
The fan had gone to the One Piece exhibition gallery and in the Jump One Piece insert art gallery he saw that particular cover page and the three sake cups. He asks Oda if "That person", (meaning Sabo), visited Ace's grave?

Oda answers and explains that for the weekly chapters the other cups were covered by text and they put up the clean version without the text for message, confirmation of "that person". Later in the manga the fans can confirm this clearly, but as for this message he'll leave for our imagination what it means.

Well the message of Sabo being alive is pretty obvious and Oda wants to still be a bit "mysterious" about it even though he basically confirmed him being alive.