It is worth mentioning that
@Snog 's comment that the prefix being displayed seems to indicate that the prefix still exists is correct. If the prefix was deleted within XF (and nothing interfered), related elements would've been cleared up so it shouldn't appear. If the prefix was changed to no longer be associated with the forum in question, then the prefix would still appear on existing threads (though it would be changed on edit I believe).
Regardless, the code to handle this is the same for both situations.