One of the functions of GNU Screen is to lock the screen.
The man page says this:
Lock this display. Call a screenlock program (/local/bin/lck or /usr/bin/lock or a builtin if no other is available). Screen does not accept any command keys until this program terminates. Meanwhile processes in the windows may continue, as the windows are in the `detached' state. The screenlock program may be changed through the environment variable $LOCKPRG (which must be set in the shell from which screen is started) and is executed with the user's uid and gid. Warning: When you leave other shells unlocked and you have no password set on screen, the lock is void: One could easily re-attach from an unlocked shell. This feature should rather be called 'lockterminal'.
If I login to a Linux console start screen and then lock the session and then walk away is there any way a person will be able to quit out of screen and use the system under my credentials. The above section from the man page isn't entirely clear to me. Assume that attacker has no other access to the system and they won't reboot the system.