As you might have noticed, Unity logs your history to bring up frequently used programs faster than others. This is really helpful most of the time. However, it is awkward to have some private images showing up, when your laptop is connected to the projector.
I always clear my Zeitgeist history before I connect my laptop to the projector. I use the following commands:
rm ~/.local/share/zeitgeist/activity.sqlite #this would delete the history
zeitgeist-daemon –replace #restart the zeitgeist daemon