[Dafni]

Well, if you just plug it in and let it sit, it should definitely not get worm. When you drove it, however, it will get a bit warmer when you let it sit. Reason being the sudden lack of airflow. Heat gets not generated in the can, but inside the motor. And it get's only cooler on the outside, where it has contact with the air. If your truck moves, air flows over the can and displaces heat. But when you stop, the heat distribution in/on the motor gets more even (magnet cools down, while the can might get warmer)
No new heat is generated, though, it just moves to the cooler area.

Hope this helps
DAF