Mindstorm NXT ultrasonic sensor

I downloaded the NXT hardware developer kit that was released by Lego today (http://mindstorms.lego.com/Overview/NXTreme.aspx ). I'm trying to make this
sensor work using NBC. I have the basic functionality working, but am confused by one area of the sensor doc. It says the sensor "will measure distances for up to 8 objects" and store them in corresponding measurement bytes that can be queried. Does anyone know how you can determine whether or not these 8 bytes contain the distance to an object or not?
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