I very seldom go to the PTC Community Home page, but when I did, this was waiting for me. Lukas had replied several hours earlier, and Stephen about a week ago. Why would this still be there after I have already gone to the topics from the little "bell" on the top of the page?
Today it still says "since you were gone", so it must be a function of time since the post versus and actual last visit - or "since you were gone".
Jaime Lee may be able to confirm this, but my assumption is this component is indexed after X amount of time and is not dependant upon login/log out. It is also probably more useful for community members that are newer and more likely to land on the homepage, and those that do not know how to check their notifications. Not so much for you veterans. I hope this helps!