At home, it’s not really happening that often but in public areas like at university: Being urged to leave the laptop unattended. First solution that might jumps into one’s head: Locking the screen to avoid others to mistreat your personal workspace and hack all your account information and so on.
This might be easy to do on Linux as well as on other operating systems (simply via hitting Ctrl+Alt+Del or running xlock4 in a terminal) – but you still can directly bypass the dark screen via accessing the kernel console via Ctrl+Alt+F1 (and/or other F-keys)!
Gotta research a little more on that!
Edit: Okay, though one might be able to switch to an other console instance, it’s needed to log in immediately after doing this! So yeah, it’s not a security issue after all, I just had the feeling it was like so..