Sword Art Online: Mother's Rosary, Vol. 3 (manga)

After Yuuki and the rest of the Sleeping Knights
shared a victory over the Floor Boss with Asuna and left their names on the
Monument of Swordsmen, Yuuki mysteriously disappears. When Asuna attempts to
message her, she finds that she's offline. Desperate to see her once more, she
turns to Kirito, who happens to know exactly where she is and gives her an
address. But she may come to regret indulging her curiosity...