This might be because of some program that you have/had running at the moment you turned off the monitor.
Some programs are programmed on purpose to disable the activation of the screen saver.
For example, a program that does media playback may do this to avoid disruption when watching the movie.
If the PC is part of a domain, you cannot change it; otherwise, see the instructions below.
The location of the GPO that controls this would be: Local Computer Policy\User Configuration\Administrative Templates\Control Panel\Display and more specifically;
"Hide screen saver tab".
Group Policy Editor is available only in Windows 7 Enterprise, Pro and Ultimate.
You can also change registry settings ... which I do not suggest if you don't have permission. You can get into trouble if they find out you're circumventing their policies.
You can find it here:
HKEY_CURRENT_USER\Software\Policies\Microsoft\Windows\Control Panel\Desktop
Delete the ScreenSaverIsSecure
value.
If the greyout comes back after making registry change, you may have to use Group Policy Editor to make a permanent change.
Open the group policy editor by running this command:
gpedit.msc
and navigate to Local Computer Policy\User Configuration\Administrative Templates\Control Panel\Personalization
double click the "Password protect the screen saver" entry
Tick the enabled radio button and hit OK. Close the editor.
Best Answer
You can 'Lock' your screen when you leave (ctrl+alt+del) to be sure that whoever uses the PC next has to log in.
There are a few things which can stop a screensaver from locking a session, possible windows/program updates may interrupt it... Or even a program waiting for a response/notification.