The current Zipkin instrumentation generates tracing IDs during client requests (in com.github.kristofa.brave.ClientTracer.startNewSpan()). However, it does not do so for server requests (see ZipkinTracer.ZipkinRoutePolicy.onExchangeBegin()). This means that if the client does not supply its own tracing headers, nothing will be traced until a client request is made from within Camel.
This can be demonstrated using the camel-example-zipkin modules by removing the Zipkin instrumentation from the client module. When this is done, the service1 module will not report any spans.
The Zipkin documentation indicates that tracing information should be generated when none is present: "When an incoming request has no trace information attached, we generate a random trace ID and span ID."