Allow Mod Comments to be Edited/Removed + Mod Comment Log/Record
Implementing a flagging/warning system would probably be too complicated, so in the meantime I regularly add "Keep an eye on" messages via Mod Comments as a reminder to myself and also to inform other Mods that a user has been flagged because they caught my attention - just not negatively enough to warrant banning them right away.
It would be nice to have a Log/Record of Mod Comments being added to a User Card because there's no way of knowing unless a Mod specifically communicates that they did so, or you happen to find out by opening the specific User's Card by chance. In chat alert like other Mod Actions is probably not necessary since no one is being banned yet, but at least within the Mod View's "MOD ACTIONS" widget would be nice.
There are times when first impressions are incorrect and with a polite reminder/warning some folks change and turn out to be a positive part of the community. It would be nice to be able to delete the Mod comment(s) to clear their record. Being able to check which accounts have Mod comments on their User Cards would make it possible to follow up on matters. With that ability, being able to mark Mod comments as positive/negative would be helpful because not all Mod comments are keeping track of negative things.

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BleuBelladonna commented
To add to this, an editing feature would be helpful - edit existing comments, add additional notes on a person, or updates (i.e. record of name changes, alternate accounts, etc.)