Remus had fixed up Sirius’ flat for him. Remus has been in England since before Sirius was released from St. Mungo’s, early enough that he had the time to come to the flat and clean up five years of disuse and whatever messes Sirius had left untouched when he went to check on the Potters. Remus had not only known about Sirius being released and in St. Mungo’s, he had also made the decision to not visit, but instead to secretly help Sirius settle back into society.