Check your index template
I don't think it's connected to index as it doesn't even appear there, check showthread template(s) instead. Look for weird code in Diff Report, like "hi" or language variable or post your temlates here if you don't know what you should look for.
Also run the File Verification tool in ACP -> Tools & Maintenance and switch any files that were changed to default from clean MyBB package of your version. It may be a modified file, for example showthread.php
Its not in index template. Even I guess its not in any template.
Looks like it is coming somewhere from showthread.php.
1. Try restoring the original showthread.php.
2. Check any plugin you are using related to showthread (thread display).
It's showing up in profiles too