Do you think it is a harmless prank to mess with another persons account? What would you do if you find a computer was left on and accounts were not logged out? What would you want someone to do if you left your computer on and accounts open?

No, I do not think that it is a harmless prank to mess with another person's account because what you say or do on their account can affect them in a way that you can't imagine.  If I found a computer was left on and accounts were not logged out, I would log them out myself because I wouldn't anybody messing with my account.