Plenty of small GPRS equipped self powered tracking modules around, designed to be attached to the frame of a vehicle, and using cell tower triangulation to locat themselves to a pretty good accuracy. They often do not use a GPS module, as they typically are under the vehicle and have no sky sight available, but the tower locator is generally good enough. Power is typically good enough for a week of use, and they send a packet every few hours when stationary and then when they detect motion, giving you the ability to track the path taken.
Slightly different ones are wired into the OBD socket, actually replacing the original purple one with a new socket, so they plug in to get both parasitic power and data lines, while appearing to not be there to a casual look. Those typically will have a GPS module on the top side, and are designed to be mounted under the dash or inside the plastic of the A pillar. They also do GPRS and cellphone triangulation in addition to the GPS locating, and some models are able as well to disable the vehicle ignition system remotely as well. They also have a small back up battery to enable them to send a few location packets if the vehicle battery is removed or goes flat.
Used by financial institutions to track vehicles either as part of a loan agreement, or installed by the owners to allow asset tracking and recovery if the vehicle is hijacked or stolen.