Reading Anna's answer here on legitimate reasons why moderators may flag posts and (possibly) mark them as helpful, I was interested to see what would happen for comment flags.
So I happily flagged a couple of comments with the other message of
testing to see if this could be used in a similar way as Anna suggested to leave mental notes or messages to other mods for advice without having to open up the Teachers Lounge and interrupt the serious unicorn research ongoing.
I was a little surprised to see that the flag was silently marked as helpful, but even more surprised to see that the comment was automatically deleted by the system. When I check the flag logs, I think it is because my "other" flag was actually recorded as an "offensive" flag causing auto deletion (see log).
I don't know if this is intended behaviour or not but I thought I'd ask the question, particularly with the flag reason matching up. Pity we can't use comment flagging to communicate as we might do with post flagging - if a user is being abusive in comments to a moderator perhaps and it would be more diplomatic for another mod to come in and deal with the situation.
Edit: Having just trawled the site for an obsolete comment to flag, that also got recorded as "offensive" with a message of "obsolete" and auto-silently-deleted.