Seems I read your second "somebody" posting first. Sorry about that. See there for more comments.
In this case, only the second sentence with "anyone" in it would normally be used. It is considered the correct response.
If you used the first, it would sound defiant; you would be considered to be parroting the question words back in the face of the questioner. "No, I am
This seems to be more difficult than I thought. Actually, I thought one would use "anybody" if one wanted to be defiant. "No, I am not seeing ANYBODY." and even if one wanted to joke about it. I think using this properly needs a kind of knack that non-native speakers like me lack. Can you think of other contexts where this somebody might be used?