When you visit cities you will explore various objects, but to have access to them, you have to find them first. When you pass e.g. a job agency or a truck dealer such information will be displayed on your GPS.
Such unknown objects are marked with a question mark icon. To explore them simply pass them and the icon will change. There are two types of objects to discover: truck dealers and job agencies. [A map with truck dealers and job agencies can be found in "Truck dealers" and "Employees" chapters.]