Top Hat Boy

I was once in my house on my own, my parents had gone out for the night, when I walked through the hallway into the living room and I saw this little boy standing over my dog, and my dog was looking up at him, he was sort of stroking my dog, and he was wearing a top hat. I doubled checked and he was gone, but my dog was still looking up.

Then a couple of days later, my brother was in the house, talking to his friend over the computer, he put his web-cam on, then got up to go to the loo, and when he came back his friend, who he was on web-cam, said "who is that little boy?" (thinking it was like my brother's little cousin or something) when actually, my brother was home alone. His friend said he had seen a small boy wearing black wander into the room, appear on web-cam, and then disappear.

My house is brand new, so I can't figure out why it would be haunted, but it is built where a mental hospital was used to be built. what is going on?