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...