Frans noticed this bug while coding a new style, I was able to verify it.

There is a variable set in the ucp_pm_viewfolder.html
messagerow.S_TOPIC_REPORTED that is not set anywhere in the PHP code, so this would always result in false.

<!-- IF messagerow.S_TOPIC_REPORTED --><a href="{messagerow.U_MCP_REPORT}">{REPORTED_IMG}</a><!-- ENDIF -->

It is used in mcp_forum.php, search.php and viewforum.php but not in any UCP file or overall functions file.