Automatic Security Updates are disabled by default as they tend to put strain on the system at times the user may not want or be running when they're trying to install other software using synaptic or appgrid. This causes an error to be reported that the process is locked by another service. This can be confusing to a user where they think something is wrong with the system. Even more so when the system is taking a long time to shutdown because it's installing updates.

To reenable simply open a terminal and enter the following..

sudo apt-get --purge remove unattended-upgrades

sudo apt-get install unattended-upgrades

Two files  20auto-upgrades and 50unattended-upgrades should be created in /etc/apt/apt.conf.d

If not simply run the following in the terminal and select yes.

sudo dpkg-reconfigure -plow unattended-upgrades