php - redirect() is giving header() error even though nothing is being output to the browser

So basically, I'm getting an error message which reads:

Cannot modify header information - headers already sent by (output started at D:\xampp\htdocs\star\application\controllers\process_login.php:1)

I know what is the meaning of that error but I can't figure out where the output was started. I've no whitespaces in the process_login.php file nor anything echo-ed out as well.

I access the login form via the http://localhost/star/index.php/star URL

star Controller

class Star extends CI_Controller {
    public function index()
    {
        $this->load->view('login');
    }
}

On form submit, I'm posting to the process_login Controller.

process_login Controller (it doesn't even have a closing tag to avoid whitespace)

class Process_login extends CI_Controller {
    public function index()
    {
        $this->load->library('form_validation');
        $this->form_validation->set_rules('userid', 'Username', 'required');
        $this->form_validation->set_rules('password', 'Password',
        'required|callback_check_valid['.trim($this->input->post('userid')).']');
        if ($this->form_validation->run() == FALSE)
        {
            $this->load->view('login');
        }
        else
        {
            redirect('dashboard'); // this is the problem area
        }
    }
    public function check_valid($pw, $un)
    {
        if($un)
        {
            $this->load->model('user');
            if($this->user->is_authenticated($un, $pw))
            {
                return true;
            }
            else
            {
                $this->form_validation->set_message('check_valid',
                  'Invalid login. Please try again!');
                return false;
            }
        }
    }
}
/* End of file process_login.php */

dashboard Controller

class Dashboard extends CI_Controller {
    public function index()
    {
        $this->load->view('admin_area', array('page_title'=>'Dashboard');
    }
}

I'm assuming the process_login.php:1 means the output started from Line 1 of that file? If so, I don't have any output or whitespace in that file. Then why is it that I'm getting the error?

Debugging

After removing everything from the process_login.php file, I'm still getting the same error. This is what the stripped down version of the file looks like:

class Process_login extends CI_Controller {
    public function index()
    {
        redirect('dashboard');
    }
}

I'm starting to think the problem might be in some other file which are being loaded before this controller file. Hence, it's saying that the output started from Line 1.