There are two kinds of GPS- Trackers. Battery operated or connected to a power source.
Battery operated can run up to a month on a single charge and be placed anywhere in the car, but usually, they have strong magnets attached to them and are outside of the vehicle. Just look around and see if there is a matchbox size box anywhere; it should not be. They are used by jealous spouses, who have regular access to the car and can recharge the device when needed, or for short term surveillance.
Power connected is installed when someone had access to the inside of a vehicle for some time. They can be plugged in or wired, usually used as theft protection, by taxis, services, or security-aware owners.
Plugged-In Usually, Are 4 Types
Service port plugin, such as OBD, is famous. Find where your car service ports are and check if there is anything plugged into them. Note that there might be more than one service port.
Light plugin, a small fixture that clamps around a light bulb inside, front, tail, turn, or any other light. Check every light bulb in the car to see if there is anything extra attached to it.
Fuses. For around 40 USD, anybody can get a fuse with a GPS unit inside. They are easy to install, but sometimes have trouble getting GPS signal thru the hood. Just unplug every fuse and see if there is a sim card slot underneath.
12V socket. Lots of cars have 12 to USB adapter plugged in, instead of lighters. Those adapters can have built-in GPS trackers, or even a USB charging cable plugged to them have a GPS module. Just check for the SIM card port.
Wired in are multiple types.
In modern cars after 2012, if you have a touch screen computer as an entertainment system, you have a GPS build in them.
Most Car alarms have a GPS unit inside.
GPS units can be soldered anywhere to any harness in the car.
All GPS Trackers Use Two Types Of Signal
1) GPS itself that they get from a satellite, but they do not transmit anything to them. It is a total passive module. While it can be clocked or jammed, it is the satellite signal that pushes, and it is illegal.
2) The message that sends the GPS coordinates to whoever is keeping tabs. In most cases, that is a cellular network signal and tracker is using a regular sim card to send automated SMS. In some cases, it could be a satellite network or UHF. This signal is active and can be detected. The problem is that cellular network signals are all around us, and a cheap bug detector from eBay will not help; it will pick up any message around. The car needs to be isolated in an underground garage with shielded walls, like a shipping container. Still, there is a catch, this will block the GPS signal as well, and in most cases, if the unit does not get the GPS signal, it does not send anything and, therefore, cannot be detected.
Let say you fund a place in the center of Nevada desert that does not have any signals beside your car. Since the data remains a short SMS message, the vehicle has to get monitored for hours.