Thanks for the reply Chris, I checked further with the member that alerted me to this and it doesn't look like they were talking about the sidebar after all - they meant the latest thread link on the forum list.
From what I understand they've done, they "ignored" a member, then saw a thread title was the latest thread in a forum with a reply from a non-ignored member (on the homepage, next to the forum title). When they then viewed the forum section, they could not find the thread. I'm not sure if this is as intended or not, as the latest post may be by a non-ignored member, but that thread wouldn't show in the thread list.