On some of our sites /404.html is being requested by different IP addresses, every second or so. The request is not accompanied by other resources loading on the page and will quite often then alternate to request /favico.ico.
No referring URL is present and the traffic is coming from different user agents, status codes are usually 200 or 304 - the file has been requested nearly 5000 times this month.
The 404.html page is not loading as a result of an error, a bad request or as a result of a rule in .htaccess, it is being requested.
Is this a new form of attack?