I can verify this behaviour on one of our new sites also. Pages which fail to find a record in the initial query are redirected to a 404 page. Error logs still fire off though so I'm getting several each day from site sniffers.
Came on today to try to find some info about this. :) Was going to ask if it was possible to get the error log to record the referring URL string as well as the page failure. I am thinking that users are arriving from mal-formed links on other pages but I have no way of knowing what page was sending them to the one throwing the error. I think this would still be useful and I'd love to know how to add this if possible.
Discovered that the error log in CMSB itself does capture the whole server request array. Found what I needed there.
J.