Webchances are the gps unit was put by a previous dealership before carvana, and it's just been left over in the car and not seen by anyone. Some trackers are magnetic and can be quickly placed under a car. Although it's less common due to the risk of heat damage, some might be placed.

You might want to use a mirror to check the undercarriage without lifting the vehicle. Just google it and you even get the carvana installer instructions.