Remus remembers a sleepy Saturday morning, when Sirius crawled into bed after ten days doing work for the Order back during the first war. Sirius had crept into their room, as quietly as Sirius ever did anyway. (Which is to say, Sirius had tripped trying to take his trousers off on the way to the bed and a long string of curses had escaped him.) He squinted against the sun, though, smiling down at Remus and running his calloused fingers over the several days of growth on Remus's face.
"Where've you been?" Remus slurred, only half awake. It had been a long week, longer still when you added worrying about Sirius on top of his other burdens.
"Work," Sirius said simply. He wormed his way under the blankets, pressing close to Remus.
Remus made room for him, let Sirius curl into the warm spot where he had been laying. "You didn't even say goodbye, just disappeared."
"I came back," Sirius told him. He kissed the corner of Remus's mouth, the edge of his jaw, the soft spot behind his ear. Sirius nuzzled there a moment, his breath soft against Remus's throat. "I'll always come back, Moony, I promise. It's only when I do say goodbye that you need to worry."
Remus remembers fifteen years later, alone in another sunny bedroom meant for two. Sirius had still never said goodbye to him, but Remus knew he wasn't coming back this time.