E_NOTICE error in access hook

Well yes, in theory you still need it. In the case where there is no sid parameter in $_POST for instance, if ($p->sid) will throw an E_NOTICE, whereas if (!empty($p->sid) won't. You could also use if (isset($p->sid) if you prefer.

I found this access hook was being called on every page load by the breadcrumb building function, but I'm not quite sure why. Here's the backtrace from an ordinary page request with nothing at all to do with swfupload:

array
  0 =>
    array
      'function' => string 'swfupload_upload_access' (length=23)
      'args' =>
        array
          empty
  1 =>
    array
      'file' => string '/home/user/myproject/includes/menu.inc' (length=38)
      'line' => int 452
      'function' => string 'call_user_func_array' (length=20)
      'args' =>
        array
          0 => &string 'swfupload_upload_access' (length=23)
          1 => &
            array
              ...
  2 =>
    array
      'file' => string '/home/user/myproject/includes/menu.inc' (length=38)
      'line' => int 662
      'function' => string '_menu_check_access' (length=18)
      'args' =>
        array
          0 => &
            array
              ...
          1 => &
            array
              ...
  3 =>
    array
      'file' => string '/home/user/myproject/includes/menu.inc' (length=38)
      'line' => int 1026
      'function' => string '_menu_link_translate' (length=20)
      'args' =>
        array
          0 => &
            array
              ...
  4 =>
    array
      'file' => string '/home/user/myproject/includes/menu.inc' (length=38)
      'line' => int 1015
      'function' => string '_menu_tree_check_access' (length=23)
      'args' =>
        array
          0 => &
            array
              ...
  5 =>
    array
      'file' => string '/home/user/myproject/includes/menu.inc' (length=38)
      'line' => int 953
      'function' => string 'menu_tree_check_access' (length=22)
      'args' =>
        array
          0 => &
            array
              ...
          1 => &
            array
              ...
  6 =>
    array
      'file' => string '/home/user/myproject/includes/menu.inc' (length=38)
      'line' => int 1559
      'function' => string 'menu_tree_page_data' (length=19)
      'args' =>
        array
          0 => &string 'navigation' (length=10)
  7 =>
    array
      'file' => string '/home/user/myproject/includes/menu.inc' (length=38)
      'line' => int 1593
      'function' => string 'menu_set_active_trail' (length=21)
      'args' =>
        array
          empty
  8 =>
    array
      'file' => string '/home/user/myproject/includes/menu.inc' (length=38)
      'line' => int 1628
      'function' => string 'menu_get_active_trail' (length=21)
      'args' =>
        array
          empty
  9 =>
    array
      'file' => string '/home/user/myproject/includes/path.inc' (length=38)
      'line' => int 188
      'function' => string 'menu_get_active_title' (length=21)
      'args' =>
        array
          empty
  10 =>
    array
      'file' => string '/home/user/myproject/includes/theme.inc' (length=39)
      'line' => int 1844
      'function' => string 'drupal_get_title' (length=16)
      'args' =>
        array
          empty
  11 =>
    array
      'function' => string 'template_preprocess_page' (length=24)
      'args' =>
        array
          0 => &
            array
              ...
          1 => &string 'page' (length=4)
  12 =>
    array
      'file' => string '/home/user/myproject/includes/theme.inc' (length=39)
      'line' => int 697
      'function' => string 'call_user_func_array' (length=20)
      'args' =>
        array
          0 => &string 'template_preprocess_page' (length=24)
          1 => &
            array
              ...
  13 =>
    array
      'file' => string '/home/user/myproject/index.php' (length=30)
      'line' => int 36
      'function' => string 'theme' (length=5)
      'args' =>
        array
          0 => &string 'page' (length=4)
          1 => &string '<div id="homepage">
  <h2 class="feature-showreels">Featured</h2>

  <div id="homepage-featured">
    <div id="main-content-flow" class="ContentFlow">
      <div class="loadIndicator"><div class="indicator"></div></div>
      <div class="flow">
              <a href="/people/672/showcase" class="item"><img src="http://myproject.rikako.abelard.wgtn.cat-it.co.nz/sites/default/files/imagecache/player_thumbnail/DHoff.jpg" alt="Dillbox Test" title=""  class="content" width="310" height="286" /></a>
   '... (length=5605)