Yeah, you're right. That's kind of cool actually.
I know that the closed envelope means there's no new content since you visited last. The dot indicates you've contributed. The lines indicate the thread is hot. The Yellow paper open one indicates it's a hot thread with new content. The white paper open one indicates it's a regular thread with new content. The lock symbol appears on the threads that are closed.
You discovered a quick way for us Mods and Admins to lock a thread.