Google chrome session expire=null not working
This is a fairly wild guess, but I wouldn't be too surprised if it's this. Google chrome will keep running in the background if you have any extensions that make use of this installed. If that's the case after a log off-log in the session should be reset.
If that isn't it, could you please open the developer tools (cmd+alt+i) and copy all the information about the cookie from there (resources->cookies->yourdomain.com). (Especially what's written in the Expires column, because it should say Session)