|This individual may have been a good employee to date, but what he did was not good at all. |
There are two particularly troublesome aspects to his escapade. First, if he absolutely needed to access this document and could only do so on your computer, he should have called you and asked for permission. Secondly, if his intentions were honorable, one has to wonder why he closed the door to your office.
As a manager, you have private, personal, and confidential information that may have been compromised here. For him to enter your office and access your computer files without your permission shows either incredibly poor judgment or highly suspicious behavior. Although the situation is not your fault, you should consider logging off or having an automatic log-off when you are away from your office.
You may have a terminable offense here, but that decision is yours. At a minimum, you should document exactly what happened, have your computer checked out, and advise your good-to-date employee that if there are any further infractions, he will be good-to-go.