Basically, For the G2's the temperature sensor is a virtual sensor whose temperature is calculated based on several factors like temperature value from other sensors and PoE output. Fan speed will be increased accordingly as per this calculation.Algorithm used for calculating temperature will vary for each hardware variant to ensure that the system (including board devices and Power Supply) does not overheat.
So most of the time it will be the CPU temperature but also PoE drivers can become a important heat source.