Comment 4 for bug 433827

I think that the "pac+" scheme is added by libproxy0:

From libproxy/src/plugins/gnome.c:
 // Mode is wpad:// or pac+http://...
 else if (!strcmp(mode, "auto"))
 {
  char *tmp = gconf_client_get_string(client, "/system/proxy/autoconfig_url", NULL);
  if (px_url_is_valid(tmp))
   url = g_strdup_printf("pac+%s", tmp);
  else
   url = px_strdup("wpad://");
  px_free(tmp);
 }

I don't know if it is libproxy0 that exports that value to the environment without removing the pac scheme though, or some gnome component.
I also don't know if keeping the "pac+" scheme is the right thing to do or not... But it might be a good idea to notify the libproxy0 developers to see this bug report.