Correct HTTP status code for existent resource, but non-existent entity?

Question

Say the client is requesting the following URL:

/user-details?user=123

If /user-details was a non-existing resource, the corre

Answer 1

The user parameter is part of the resource identifier as stated in RFC 3986, section 3.4:

The query component contains non-hierarchical data that, along with data in the path component (Section 3.3), serves to identify a resource within the scope of the URI's scheme and naming authority

Hence, 404/Not found is perfectly fine.