...a post highlighted itself somehow (an asterisk or similar) to indicate...

I would suggest to tag an updated node using CSS class attribute (i.e. updated) with a sensible site-default:

<div class="$AS_BEFORE updated">
...
</div>
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Now, every monk can fine-tune the layout using own CSS redefinitions (Help for Display Settings). Something like i.e.

.updated { border-left: 1px solid red; }
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or

.updated::before { content: "UPDATED: "; }
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Perhaps add ud-$YYYYMMDD or updated-today or similar for more customisation options?