not2easy - 5:44 am on Jan 2, 2014 (gmt 0)
I would need to do a lot more checking to be certain, but it looks like the "nothing from 157.5n" holds true for the 404s I see also. After looking at a few hundred 404s in half a dozen log extracts I pulled a random one from last January and there was a few there which had never existed, not even complete filenames. The entire request was for "GET /no" and another for "GET /sitemap " maybe they got distracted by something shiny in the middle of the request. Those are rare oddities.
I am not seeing any 404s from files that had existed or do exist in another folder - or confused paths. I see completely unrelated image and folder names that I have never seen or used. None of the file structure exists on any of my sites and they don't request all images from one structure, they make up a new string of directories that don't exist and then invent an image filename at the end.
some actual examples:
(example.com replaces a not very nice .com domain name)
While these domains I'm checking logs for are on a shared server, they are hosted within my reseller range and there are no subdomains. Each domain has a unique IP.
I was not seeing this until the last half of 2013 but on some domains there was a ridiculous number of requests and repeated requests for non existent files that had returned many 404 errors already. On those domains I blocked them by UA. 403 does not tell them anything yet. The UA is also Disallowed in robots.txt too. I wish I could find anything helpful. Searching around I see that others complained about this problem 4 - 5 years ago but I don't see any explanation.