ASP.NET MVC: CSS file returning a 302 error when it exists

I'm getting a 302 error returning on a single CSS file on an ASP.NET MVC 2 site in localhost this morning and I don't know what would have changed to cause this.

The localhost site uses IIS 7.5, though I've had limited experience with IIS so I haven't looked to much in to what could be going on there.

The URL to the CSS file is:

http://localhost/MySite/Content/Site.css?v=16

and the location header on the response looks like this:

/MySite/Account/Login?ReturnUrl=%MySite%2fContent%2fSite.css%3fv%3d16&v=16

This makes me think that MVC is redirecting the static file or something like that, however if that was the case, then I would expect all my images, CSS and JavaScript files to be doing the same which they're not. Just in case, here is a simplified version of RegisterRoutes() in Global.ascx:

public static void RegisterRoutes(RouteCollection routes)
{
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

    routes.MapRoute("", "Account/{action}/", new { controller = "Account" });
    routes.MapRoute("", "{action}", new { controller = "Home", action = "Index" });

    routes.MapRoute(
        "Default", // Route name
        "{controller}/{action}/{id}", // URL with parameters
        new { controller = "Home", action = "Index", id = UrlParameter.Optional } // Parameter defaults               
    );

    routes.MapRoute(
        "Error",
        "{*url}",
        new { controller = "Home", action = "ResourceNotFound" }
    );
}

Also, if I change the name of my CSS file to Site2.css and reference that instead, the same thing happens.

What's going on?