sorry, can you be more clear about which page view type you are looking at?  group, forum or topic?

there are default icons on group view (list of forums) for forums with new posts…  sp_ForumIconPosts.png

additionally, on group view, in the forum status section, there is an icon to indicate unread posts…  sp_ForumStatusPost.png

there are default icons on forum view (list of topics) for topics with new posts…  sp_TopicIconPosts.png

additionally, on forum view, in the topic status section, there is an icon to indicate unread posts…  sp_TopicStatusPost.png

and you can also define custom icons for both of those on a forum by forum basis (like we do here) by uploading the custom icons on forum – forums – custom icons… and then applying them to the desired forums on forum – forums – manage groups and forums…

if you are referring to something other, please clarify…

Ricki Steigerwald said
…an icon which appeared next to topics that the user had not posted in since the last comment.

Perhaps you’re referring to the two Forum icons in Group view:

• sp_ForumIcon.png
• sp_ForumIconPosts.png

The latter displays for the user if there are unread posts.

There are two similar Topic icons in Forum view:

• sp_TopicIcon.png
• sp_TopicIconPosts.png

I’ve attached two screenshots of ours in use, one for forum view, the other is the topics list.

Is your SP theme up to date? Have you customized the theme? You may just need to update the SP theme, or check the codex for changes if your theme is customized.

Hope this helps!