Some trackers are magnetic and can be quickly placed under a car. Just google it and you even get the carvana installer instructions. Although it's less common due to the risk of heat damage, some might be placed.

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. You might want to use a mirror to check the undercarriage without lifting the vehicle.