[TouchingVirus]

The highlight code appears in functions_bbcode.php at line 203..i think this is what you were looking for..

PHP Code:

// [HIGHLIGHT]
    $bbcodes['custom']['find']['[highlight]'] = '#\[highlight\](.*)\[/highlight\]#esiU';
    $bbcodes['custom']['replace']['[highlight]'] = "handle_bbcode_parameter('\\1', '" . str_replace("'", "\'", '<span class="highlight">\1</span>') . "')";
    $bbcodes['custom']['recurse']['highlight'][0] = array('replace_html' => "<span class=\"highlight\">\\7</span>");