Do you have the sensing package? If so, it's the lane keep assist. If you don't have the sensing package I don't know what it is used for.

 

"Back" as in toward the back of the car? Or "back" as in facing forward?

There can be sensors on both sides. Lane cameras toward the front... Mirror dimming sensors facing rearward.

 

The back of the mirror facing forward. Didn't think the lane camera would need sensors since its activated by the turn signal or button. If the sensor is facing forward then it's not really detecting the side lanes.

I don't have Honda sensing.

 

it's an auto-dimming rearview mirror sensor

 

Yup it's the auto dimming for the rear view mirror. When I placed my finger over the sensor I can see the mirror getting darker within a couple seconds. When I cover the back sensor and place my phone flashlight on the two sensors on mirror side I see it get even darker. The lane watch and rear view camera doesn't seem to be affected. I tested in daytime and headlight on auto and it doesn't trigger the headlight. So as far as I know the front and back sensors on the rear view mirror is just for the auto dimming.

 

My rearview is unplugged and the car still senses the temperature inside the cabin.