Not sure if this affects other things, but at least with the requisitions API, when I request a requisition that doesn't exist, I'm getting a 204 (NO CONTENT) rather than a 404 (NOT FOUND).
AFAIK, 204 is only for when the person made a valid request that matched an entity, but that entity is empty.
$ curl -L -o - -u admin:admin --verbose http:* Trying ::1...
* Connected to localhost (::1) port 8980 (#0)
* Server auth using Basic with user 'admin'
> GET /opennms/rest/requisitions/foreign-source-that-does-not-exist HTTP/1.1
> Host: localhost:8980
> Authorization: Basic YWRtaW46YWRtaW4=
> User-Agent: curl/7.43.0
> Accept: */*
< HTTP/1.1 204 No Content
< Set-Cookie: JSESSIONID=1r8pksij9kq642ktmjxyxf2jf;Path=/opennms
< Expires: Thu, 01 Jan 1970 00:00:00 GMT
< Date: Tue, 09 Feb 2016 21:05:37 GMT
< Content-Length: 0
< Server: Jetty(8.1.10.v20130312)
* Connection #0 to host localhost left intact