The case is actually not modified when a note is added that is why the date_modified is not changed.
You could create a before_save logic hook on Notes that update the date_modified on the related bean (case).
If you are new to logic hooks use the tool in my signature to create an after_save logic hook on Notes and put something like this inside it.
PHP Code:
if ($bean->parent_type == 'Cases')
{
require_once "modules/Cases/Case.php";
$case = new aCase();
$case->retrieve($bean->parent_id);
$case->date_modified = $bean->date_modified; // set case date same as the note
$case->modified_user_id = $bean->modified_user_id;
$case->save();
}
M
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