一応、404はRFC 7231に"The 404 (Not Found) status code indicates that the origin server did not find a current representation for the target resource or is not willing to disclose that one exists." https://tools.ietf.org/html/rfc7231#section-6.5.4 とあるので、存在を隠したいときにも使える
tools.ietf.orgRFC 7231 - Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content
この点に関係するRFC 2616の記述は "This status code is commonly used when the server does not wish to reveal exactly why the request has been refused, or when no other response is applicable." https://tools.ietf.org/html/rfc2616#section-10.4.5 で、こちらは利用状況の解説という印象を受ける
tools.ietf.orgRFC 2616 - Hypertext Transfer Protocol -- HTTP/1.1
