I recently set up an entire GPS platform for our fleet at work. Security was an issue so I purchased the platform and run it in house on a server I built. Currently have 200 assets, but the platform will handle 5000.
They are probably using a device similar to an Enfora modem. These are cellular only, and fairly basic, although they can be configured to reports certain parameters such as ignition on, motion detection, geofencing, etc.
At the other end of the scale you can have a dual band device like the i50B which is Iridium satellite and cellular. The satellite kicks in depending on threshold setting for cellular signal strength. Of course there are MANY similar devices that run on different satellite networks (Global sat etc).
Reporting can be from every 30 seconds to once per day.
The devices are hard wired and use very little juice. You would never notice them. Both the devices mentioned are slightly larger than a pack of smokes and need power and ground. For the best reception an antenna is required, but that is also very easy to hide. Installation would be less than 30 minutes.
Interestingly, jammers are becoming a real problem. You can purchase them online, they only block the GPS frequency, and plug in to your cigarette lighter. Think Taxi cab drivers and truckers.