Even for self powered devices, the VBUS connection to the device pin (if any such pin is available in the device) is recommended for detection of device connection and disconnection, as per spec. Different devices have different ways of implementing this. Some have dedicated pin, whereas some make use of GPIO for understanding the device connection/ dicsonnection. I am not sure how the PSoC device handles this. I think PSoc forum will be the best place for this question.