Ubuntu has mechanism to log user activity such as used documents. This is used to facilitate searching, but can also be intrusive to your privacy.
Here is a way to disable this logging without breaking Unity or any other part of the system, execute these commands in a terminal:
sudo mv /etc/xdg/autostart/zeitgeist-datahub.desktop /etc/xdg/autostart/zeitgeist-datahub.desktop-inactive
rm -rf ~/.local/share/zeitgeist
then log out and log in again.