I have a script that sends a time-sensitive notification to users when there is a new question directed to them. However, I found that some people leave their computers open and go grab lunch, therefore missing notifications.
I'm looking to put together a script that detects if the user is idle for 5 minutes, and if so, it would show them as 'offline' and close down notifications. 
I was curious if it is possible to detect inactivity even across tabs? (for example if a user switches to another tab to Facebook.com and stays active there, they would be seen as 'active' even though they are not on our webpage specifically).
Everything that happens when the user is NOT on your side is impossible to track (luckily).
So not this is not possible (think about the security).
UPDATE
Now that I think of it. It is possible, however very unlikely that you can do it. If your name would have been Google you would have come a long way, because lots of websites use Google analytics. But other than that: NO not possible for reasons mentioned.
Is it possible to detect idle time even across tabs?