Nitpick, but the "GET /people/{id}/exists" in the example to check if the instance exists is a bit weird, would suggest "HEAD /people/{id}" for this type of operation.
Yeah, the HEAD or GET should return a 404 on not existence; however, the HEAD also won't return data which is the point. GET implies that you have need of the returned data (presumably all the properties of that person). GET is for "it exists AND I want to know about it". HEAD really is more appropriate here.
