When requesting these "resources" the server sends back a HTTP status code 400.
Status Code 400 is defined as:
http://tools.ietf.org/html/rfc2616#section-10.4.1
"The request could not be understood by the server due to malformed
syntax. The client SHOULD NOT repeat the request without
modifications."
Since the requests are not malformed, but the resource is unknown, the server should respond with a 404 status code (or a 410 if the resource has been available before).
Example requests with unknown IDs:
When requesting these "resources" the server sends back a HTTP status code 400. Status Code 400 is defined as: http://tools.ietf.org/html/rfc2616#section-10.4.1 "The request could not be understood by the server due to malformed syntax. The client SHOULD NOT repeat the request without modifications."
Since the requests are not malformed, but the resource is unknown, the server should respond with a 404 status code (or a 410 if the resource has been available before).