So unprofessional and disrespectful for both people...
Forgery is a crime - and I think sending an email from someones unlocked computer would fall under that if it was pressed in court.
If sysadmins have things set up that way, they can unlock a locked desktop and log in as the user; or even change the users account password and log in. Using these two methods as the example I would think no sane person would think it ok to log in and send an email as described. So why is it OK in this case, when a computer is left unlocked?
A normal person should feel guilty digging in another persons stuff - always; and the person who was violated should also be indignant. Period.
What if a boss or coworker had requested you to take care of something on their computer? Is the fact they gave you the access make it ok to look through their browser history while your there? If not, what exactly is and is not off limits during an 'authorized' entry event? And what is different if they left it unlocked instead?
Hope you can see how this opens a huge pandoras box.
Also, if people are not there to protect the team and coworkers, how can they trust each other when it is really important?? What will happen when there is an attacker from the internet forging things? How will you even know who is telling the truth then?
A professional will take care of the problem with a warning or a note. After so many repeat offenses, give the warnings more teeth, and/or start tracking them and provide punishments. Alternatively lock the desktops with an inactivity timer.
Just realized maybe you are trolling (or hoping you are!).