This document covers how to secure a Mac with the Keychain application for when it will be unattended.
1. Navigate to the Utilities folder within the Applications folder. Open the Keychain Access Utility application.
2. From the application's main menu, select Keychain Access, then Preferences...
3. Check the box next to Show keychain status in menu bar. Close the window and the Keychain Access application.
4. You will now see an small lock icon on the right-side of your applications window (near the date and time and Spotlight). Click on the lock icon and select Lock Screen.
5. The screen will immediately switch to your screen saver, but you will be prompted for your password in order to get back to the desktop.