There is a thermocouple (thermometer) located in the center of the back wall of your grill that measures temperature.
Note: Temperatures may vary in different parts of the grill due to proximity of the heat source, etc.
Why is the temperature of my grill different at different points in the grill? and how can I prevent it?
All grills can have variability across the cooking grate, especially at high searing temperatures. In order to reduce this variability, it is important to keep the grill lid closed as much as possible to prevent any change within the cook box. As we say at Weber, "Looking ain’t cooking."