I just saw this post which has the following notice applied to it:
Post is related to a rapidly changing event.
What does this notice mean?
Who can place/remove these notices?
This is a notice placed by a moderator which means that this post is likely to change rapidly based on on-going real-world event(s).
Yes, it's a post notice that moderators can put on a question / answer. The idea is to give heads-up to visitors that information contained therein may change over a short period of time.
Naturally, any answer can change in correctness over time, but with things such as safety in a country due to civil unrest et al I felt it was a good use of the post notice to notify viewers of the situation.
PS - It's not a flag, it's a post notice.
-Ankur Banerjee ♦ on Travel SE (Meta).
For more information, see: What does the “Current event” post notice mean?