Your system is setup to redirect to 404.shtml (and other error pages as needed) automagically, not missing.html
You might redirect to your own file if you like, but an .html file wouldn't be as useful as the .shtml file would be. (for variables you might like to use)
Suggestion, edit your 404.shtml (and the others in your error pages section) any way you like. Backup the default pages of course in advance of editing for safe keeping.
If you look closely at the default 404.shtml you'll find this line in there (for example)
Code:
<!--#echo var="REMOTE_ADDR" -->
If you placed that line in your missing.html it wouldn't do anything for you. If you renamed your file missing.shtml it would display information to you in your browser.
These are useful for error pages. You'll notice on your error pages editor that there's several buttons across the top for Referring URL, Visitor's IP Address, Requested URL and so on.
Click somewhere in your page where you'd like to add one of those variables and then click the button to auto add one for you. Now save the edits and try visiting this URL:
yourdomain.com/infoprowashere.html
That URL should redirect to your 404.shtml page, but, in your address bar you will be here: yourdomain.com/infoprowashere.html
And of course there is where you'll see your page with the varibable you added.
GL!