Sirius was restless, and being in the Shrieking Shack, although safe because unused was also torture because of the memories which it contained - memories of four boys in their animagus forms, three friends helping the fourth to cope with what he could not change - and mostly memories of a love which had sustained him and continued to sustain him even now, for he refused to believe that Remus had given up on him.
Remus. Thoughts of him had kept him sane throughout the past twelve years. Although some might care to argue the point.
He changed into Padfoot and began to prowl the woods. And then he smelled him. Remus again, and he moved toward the scent, following it until he actually caught sight of the man, and he began to softly whimper, although keeping out of sight.