I think when she says 'leave home' she's talking about David and Alice's intent to move out. Just before Aaron surprised her from behind, she's thinking about how to fix the rift between herself, niece, and nephew. I think Alice and David left for work, which I assume they did even if we don't see them necessarily leave through the door. I think Sarah's uncle Peter is perhaps the only person who could probably walk in on them, that is, if his foot is not still in pain.