azjoe,

Just remember that it is not inactivity of the computer keyboard and mouse but the web application.

Inactivity for web applications means that you are not interactiong with the application: browsing, downloading or uploading. So if you take 20 min to type a response without clicking preview or submit your session will time out. This will cause all threads to be marked as read.

This is exactly what I have been noticing.

Another scenario is walking away before finishing reading all the posts in one sitting. Which happens often during the day.