No iRule Match, No Default Pool

I think you'll find that in most cases the virtual server will simply reset the connection, with no log messages. There is simply is no response other than a TCP reset. From an event perspective, there's a CLIENT_ACCEPTED, an HTTP_REQUEST, and then an LB_FAILED event which is followed by a CLIENT_CLOSED event. From a wire perspective, you should see a full TCP handshake on the client side, the client PUSH some data (the HTTP headers), and then an immediate RESET from the virtual server (based on the LB_FAILED event). Again, this shouldn't generate any error messages in the logs unless you've specifically set something up to catch this. There are of course MANY ways to avoid this behavior, from reselecting from a different source to generating static HTML content.