Downplaying it was expected, though Remus had his suspicions Sirius likely did have a moment or two of worry. Not that he was arrogant enough to assume Sirius would lose his mind over Remus not being there but given what he knew they meant to each other in his time, how painful it was to be apart and what it'd done to them, he knew his best friend had a tendency to get upset, and so did Remus where Sirius was concerned.
Best not to dwell upon it, maybe, although it felt like a lot of emotion had been brewing between them over James and the fear of one of them being next to go. He'd let it lie for now, because Sirius was making it very clear he wanted him to sit with him and Remus wasn't about to say no. He moved next to him on the couch, and turned so he could press his forehead to Sirius' temple. "Promise," he whispered, though he also would make sure he didn't leave without saying anything first or would definitely be back before Sirius knew he was gone. Ever so lightly, he pressed his lips to Sirius' cheek.