Along the mighty Santa Clara River are planted many lemons and once upon a time many Valencia orange trees.  There's lots of wild life along the river which causes some issues with food safety.  One of the many creatures are rats and when things dry up they start foraging further and further for food.  They really like citrus and can cause a lot of damage.  What I first thought was sun burn damage turned out to be rat feeding on the bark and cambium of these lemon trees.  The damage wasn't in the right position to be sunburn damage.  Inside the canopy, often on the north facing branch.