Whistling and the dark figure

Lastnight, after I went to the restroom and came back downstairs I seen a dark figure at the bottom of our staircase. I thought maybe it was our cat because that was about the size of it, but it didn't come all the way down where I expected to see the cat, it just disappeared. It was really dark(black) in color. This morning, just a few minutes ago my girlfriend hear whistling in our playroom before I came back from taking our daughter to school. She said it sounded like a happy whistle, like a tune that you can skip too and also that it did not sound like an adult. It went upstairs quick also.