You left your computer unlocked. You're making DeeDee's job too easy.
Yes, you should definitely lock your computer when leaving it unattended.
Why? This prevents anyone, even a little hacker like DeeDee, from stealing your data or using your device while you're away. You can even configure an automatic lock on your computer after it’s inactive for a certain amount of time. Ask your IT department about this setting to ensure your computer will lock even if you forget.
Hit the "Windows" key and the "L" key on your keyboard at the same time.
Press "Ctrl-Alt-Delete". On the menu that pops up, click "Lock".
Press "Command+Control+Q". This will lock your Mac and return you to the Login screen.