His father and mother had both left the city, and Epimetheus had been gone for some time. Atlas might have been a bit of a solitary creature, but family was still family, and he hadn't seen Prometheus in far too long. So he'd gone to see his brother, not checking in on him, exactly, but just saying hello.
He smiled slightly in return. "Hello, Prometheus," he said, stepping inside. "Good to see you." Then he peered at him. "You look tired. Not sleeping?"