Iis – robots.txt and other .txt returning 404 on IIS

You might find what you are looking for in the W3SVC logs. You can see the request as it came in and I would think that should be enlightening. http://support.microsoft.com/kb/313437

In the logs, check the substatus code for the 404 responses.

404 Substatus 2 in IIS means: Web service extension lockdown policy prevents this request.

If this is the case, you probably just need to enable the ASP.NET web service extensions. See here: 404 when page exists - IIS 5, ASP.NET 4.0

If you take a look at the URL Rewrite Module it has some default rules to enforce or change the URL. Two of them that I know about are converting the entire URL to lowercase and another appends a trailing slash to all non-file requests. They are there primarily for SEO purposes but combined with what you have might "replace" or compliment your current setup.