A ghostly voice?

I heard a clear female voice earlier saying "Yeah, I know.". I thought it was my sister, so the conversation ended up going like this.

Random female voice: Yeah, I know.
Me: You know what?
Sister: What?
Me: You said "Yeah, I know."
Sister: I did not. I said "Penny." (Penny is the name of one of the dogs)
Me: No...you said "Yeah, I know."
Sister: I didn't!
Me: You did!

I'm sure my sister now thinks that I'm completely crazy, and I'm sure some people reading this will also think that I'm crazy. I honestly did hear, in a clear female voice, "Yeah, I know.". The question is, who was it, and what do they know?

I'm thinking that it may have been a ghost because I have seen a little girl in the past stood in the doorway of my bedroom. I've no idea who the girl was, or why she was standing there. I've told a few friends about this already too. Some believe me, and I'm sure some think I'm completely crazy. But I do remember that night. I was talking to some friends online, and I needed to use the bathroom, so I got up to leave my bedroom. As I reached the doorway, I saw a young girl, who must've been about 8 or so in age. Surprised that she was there, I backed away, and bumped into my desk, at which point she disappeared. I have NO idea who she is because I've never seen anyone like that before.

I also have a photo in which a face appeared on the screen of my television. The TV was turned off at the time, so there is no reason why the face should be there. Also, I was completely alone in my bedroom when I was taking the pictures, so it's not the reflection of someone who was in the bedroom with me...at least, not the reflection of anyone LIVING.