$REDIRECT_ERROR_NOTES

Hello, This is my first post to the group so go easy on me. I am using apache httpd.conf file to redirect 404 errors to a page which basically displays and error message, amongst other things. It all works fine but... I want to be able to find out what the document that they attempted to access was named, so I can display and log the value to a special log that I am using for othger purposes. Should the 'not found' url be in $REDIRECT_ERROR_NOTES ??? This value doesn't seem to be set. And its not in any of the HTTP_ENV_VARS or HTTP_SERVER_VARS. I know the answer will be something simple but I cant seem to find much help anywhere. Any help or push in the direction of a useful web page would be nice. Paul