Suppose I have flagged any question or answer by a person with high reputation or any moderator, as inappropriate (or any other flag). As far as I know, it will be placed in the review queue. Now, anyone with high reputation or a moderator can access these queues. Then, will that person be able to see that his own post has been flagged? In that case, he will be able to decline that flag, although perhaps it was necessary to delete the post or take action against the post.
Non-moderator users have no way to know that their posts have been flagged except by the phenomena associated with the review of such flags, e.g. automatic comments with a "From Review" link at the end.
Moderators can see all flags, even on their own posts, but we consider it good form to not act on such flags and let another moderator handle them.
Their own posts will never be shown to users in a review queue, meaning no non-moderators could ever decline flags to protect their own content. If you have reason to believe a moderator is abusing their powers to maliciously decline flags that shouldn't have been declined to protect themselves, raise it here on meta or contact the Community Team.
Some points not fully covered in the existing answer:
- All reviews are public, including the (summarized) actions of individual reviewers ─ cf. this example.
- All reviews of any given post are accessible from the post timeline (a non-linked site feature) ─ cf. the timeline for the example above.
- All reviews leave a trace on the Stack Exchange Data Explorer, and it is pretty easy to build a query that will take a user ID and return all existing reviews on all their (non-deleted) posts.
- It's less useful for looking up on your own posts (and more useful for getting an overview of how the site reviews are doing as a whole) but all 10k+ users have access to the full history of the review queues, via the history tab of the corresponding review queue.
That said, all of these are for information only, and are not actionable items. Users cannot review flags that affect their posts (with the obvious exception of suggested edits); I can't find a confirmation on the mother meta (possibly because this is considered obvious) but if it happened I would class it as a serious bug which should be reported asap.