Work Log Not Logging Out...

We have the work log module that we thought gives an imperfect yet somewhat adequate overview of approximate PM time spent on projects. But I was looking at our logs and we seem to be having an issue. 

One of our PMs keeps project tabs open and at times doesn't come back to them. The work log settings as an option for "Time required to determine that user is offline (in minutes)". 

We assumed that it would mean that after five minutes of inactivity, the work log would no longer track hours.

Yesterday, the PM mentioned above, clicked on a closed project to review the details, and then left it open in an unused tab for a few hours. In the end, three hours of PM time was tracked to that PM and to that project.

Is this a bug? An error? Or are we using it wrong? Why wouldn't it time out after 5 minutes like the settings said.

Does anyone else have any experience with this? I haven't noticed this before, so is it possible that something changed with version 7?? Any suggestions? Do we need to train our PMs to close tabs after they are used so the time tracking works?

Thanks!