I created a flag a while ago (28-mar) and it still not reviewed. But the flagged post doesn't exist anymore.
Could someone check this for me? I can't open a flag to this problem =X So I'm posting it in meta.
Asked a dev about this. Yay Jarrod!
badp's answer is almost on the right of it. The source of the problem is indeed due to "convert to comment". The conversion code for handling flags will properly handle any flags for moderator attention that are cast on the post, but it will not handle spam/offensive flags. The flag remains on the answer, but then it expired over time and was auto-invalidated. The flag history page was not aware that this happened, though, so it is not showing the proper "declined" that it should.
This missing indication should be fixed at some point, as well as the behavior of the flag handling.
Well, here's an educated guess. Take a peek here at this:
What happened here is that this post received 4 flags; 3 were correctly dismissed ("deleted") as helpful by agent as he deleted the post, while yours... died 2 days ago. However, the post wasn't just deleted, it was converted to a comment.
My guess is that since comments can also be flagged as spam, the spam flag remained on the comment itself, although kind of in limbo (I certainly don't recall seeing a spam flag on that comment, or I would've dismissed it — and not as helpful, either). ← This was wrong, see here. Offensive/spam flags are simply ignored on conversion. Flags eventually expire automatically and the timestamp tooltip, 2013-03-31 03:00:05Z, suggests that this is what happened.
At any rate, please consider your flag dismissed and declined. :)