Screen locking issue

I have noticed that when I have Virtualbox running a Windows 8.1 VM. When I walk away from my PC for 5 minutes and come back my monitor has gone to standby which is normal, when I move the mouse instead of getting a prompt to login again it goes straight to the desktop again. Then after a minute or 2 of using the PC it will lock he PC and prompt for password like it should have done when coming out of idle.

ProblemType: Bug
DistroRelease: Ubuntu 16.10
Package: gnome-screensaver 3.6.1-7ubuntu5
ProcVersionSignature: Ubuntu 4.8.0-27.29-generic 4.8.1
Uname: Linux 4.8.0-27-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.3-0ubuntu8.2
Architecture: amd64
CurrentDesktop: Unity
Date: Sat Dec 24 09:36:29 2016
GnomeSessionIdleInhibited: No
GnomeSessionInhibitors: None
GsettingsGnomeSession:
 org.gnome.desktop.session session-name 'ubuntu'
 org.gnome.desktop.session idle-delay uint32 300
InstallationDate: Installed on 2016-10-22 (62 days ago)
InstallationMedia: Ubuntu 16.10 "Yakkety Yak" - Release amd64 (20161012.2)
ProcEnviron:
 LANGUAGE=en_GB:en
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-screensaver
Symptom: security
Title: Screen locking issue
UpgradeStatus: No upgrade log present (probably fresh install)