Comment 47 for bug 1568604

As for #43 still, if you use other locker than light-locker and switch user using an Action button in xfce4-panel, you can switch back without giving password (by pressing Ctrl-Alt-F7). So it is better to use a script "xflock4 && dm-tool switch-to-greeter" for switching user, if you do not use light-locker. Besides, xscreensaver does not let you unlock a guest session, since it takes null password as cancel. Customize your guest session to not lock on suspend, not to autostart xscreensaver, and to have the ~/.Xresources customization for it, too. (https://help.ubuntu.com/community/CustomizeGuestSession)

BTW. you can not unlock guest session even if you use light-locker.