All major web applications (Google, Facebook, etc.) return page status 200 ok in case of authentication failure, i.e. wrong login/password pair.
Although by definition, if a resource is not found with request URI - status 404 Not found is returned.
Wikipedia says:
[404 Not Found] Used when the requested resource is not found, whether it doesn't exist or if there was a 401 or 403 that, for security reasons, the service wants to mask
How does the login case differ?