Comment # 36 on bug 1169783 from Robert Kaiser

Hmm, and even more:
> grep "plymouth/pid" /usr/lib/systemd/system/*.{path,service}
/usr/lib/systemd/system/systemd-ask-password-console.path:ConditionPathExists=!/run/plymouth/pid
/usr/lib/systemd/system/systemd-ask-password-plymouth.path:ConditionPathExists=/run/plymouth/pid
/usr/lib/systemd/system/plymouth-start.service:ExecStart=/usr/sbin/plymouthd
--mode=boot --pid-file=/var/run/plymouth/pid --attach-to-session
/usr/lib/systemd/system/systemd-ask-password-console.service:ConditionPathExists=!/run/plymouth/pid
/usr/lib/systemd/system/systemd-ask-password-plymouth.service:ConditionPathExists=/var/run/plymouth/pid

FWIW, as I'm using the proprietary NVidia driver here, I do not get a graphical
screen for password entry but always fall back to the console, which may play a
part as well.