I don't know why or when it started. I just noticed that the laptop where I have Windows 8 has an absurdly short timeout to lock the screen. The screen that is shown is the picture of the current user prompting for password.
I have tried many things in an attempt to get rid of it. Note that this version of Windows 8 is not Pro or Enterprise and It does not have the gpedit.msc tool:
In the screen options, in screensaver, no screen saver is set, but the timeout field steel controls when the screen will be locked or turned off, so I set value 0 there, and it changes to the string "(Never)". Also I unchecked "prompt for password". But screen steel locks.
Googling I found that there is a policy, editable in gpedit (no available in this edition of Windows), to set the timeout to lock the screen. Then I looked for the registry key that sets that policy and changed it (but it seems that it has no effect at all, screen still locks with a timeout of 1 min approx). The key I edited is "HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Policies\System\InactivityTimeoutSecs". I didn't existed, so I created It as DWORD with a large value.
Machine is SAMSUNG ATIV Book 2, in case somebody knows of any software that comes by default that may be causing this. I uninstalled the trial security software and I'm using Windows alone with Windows Defender and my common sense.