Given that the Deck 5 Lounge had terrible hours and therefore wouldn't be opening for another seven hours, Lucifer was bored. So he'd wandered over to Mary's place for no other reason than he guessed she would be in her room and she'd probably let him in.
Which she was, and while she didn't exactly welcome him in, he strolled into her quarters and started looking around. "I wasn't doing anything and I figured you wouldn't be doing anything either," he announced cheerfully.
"Turns out I was right," he mused and continued inspecting the room, until he spotted the photograph of Mary and her son. "Oh, is this your spawn?" he asked, picking it up and studying it. "George, right?"