I actually have my laptop set up to take a snapshot every 20 minutes (unless I'm connected to work wifi - never know when it might be pointed at confidential data on a whiteboard, and it felt unfair to opt all my coworkers into it.)
It has definitely made me more mindful of situations like this.
(It actually sporadically refuses to take a photo on the new laptop right now, so http://lishin.org/pavelcam.jpg doesn't always get updated.)