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 it's more like a harmful prank! If I found a computer which account was left on and was not logged out, I would maybe look at the name and then log out. If I left my computer on with my account open I would kindly appreciate if the person logged out without being rude.