Nobody's here...?

This happened this morning. I was home alone waiting for my parents to come back. My sister wasn't there, so there shouldn't have been anyone else, but I kept hearing it. A voice saying, "Lonely, lonely, lonely, lonely, lonely." It sounded like a young girl, maybe 4 or 5 years old. I thought maybe she was lonely, so I started singing, and then the voice disappeared. It surprised me that something like this could happen in a house I've lived in for years.