Severus wasn't often surprised by people. This was partly because he needed to anticipate inevitable betrayal, but also because he was a deeply suspicious man to begin with. Yet a boy of only eleven had cleverly worked out that Severus was his best chance at arranging a meeting with the father he never knew, and Severus could admit he was impressed. If he'd been skeptical of Leonardo's claims before, he was fully convinced now. There was no denying that look in the boy's eyes — he was a Malfoy, and he'd get what he wanted by any means necessary.
That, of course, didn't mean that Severus was going to be careless about this. It was clear from Lucius' response that Narcissa was currently unaware, and if he didn't tread carefully, he'd soon be dragged into whatever marital dispute arose later on. Assuming Lucius survived said marital dispute.
He opened the door to his private rooms and stepped aside immediately to permit the man to enter, relieved that he'd come so quickly. "He's in the storeroom," Severus explained once the door was closed, getting right to the point. "Organizing as assigned, and unaware of your presence." The implication was clear: Lucius had time to back out if he wanted to. No repercussions.