As a check, leave your seat belt unbuckled as you drive away to see whether the sound you hear is the same as that which you hear for the phantom alert. As a test, buckle up the passengers seat belt to see if the sound is eliminated. May be a faulty sensor.
Not sure how the US models differ from other geo's as far as seat belt warnings are concerned. My view of the US PDF manual doesn't mention nor show the rear passenger warning light on the dash as we have in Australia (see pic). Plus, I don't know what many have changed with the new model you have.
For Australia, the rear seat warning will illuminate every start and, if someone is seated in the rear without fastening the seatbelt, a warning (same as the front seat DING) will sound if the vehicle moves off and the seat belt remains unfastened. This is a weight based alarm so can be triggered by a weight on the rear seats. The warning will only sound once for the first start of the trip and then not come on again unless the vehicle ignition is turned off.
Not sure if this alert system is installed on US builds.