Thanks, Ted. You raise an excellent point. This is how our organization's RESTful API manual covers the data not found condition:

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No Content

The request was successful (PUT, PATCH, DELETE, GET) but there is no data to return. A 404, Not Found, could be used for GET, but it can make coding easier if the 400 series is used only for error conditions and Not Found is not an error as long as the resource type is valid.