I don’t know of another way that message appears? I think it’s specifically an Auto-Close one that’s based on last reply. It seems that the topic had a timer put on it somehow, and by the system rather than by a mod or admin.
If you have access to the data explorer you could check the topic_timers table for clues, but I don’t know how much that will give away? If there’s no record of one for that topic that might help rule it out at least.
SELECT *
FROM topic_timers
ORDER BY created_at DESC
Has it happened to any other topics?