Mounted to the passenger side of the rear-view mirror. I would guess either automatic headlight sensor or it houses the mic for hands-free phone, or both.

I'm considering mounting an EZ-Pass transponder immediately underneath it, but I'm wondering if it will block the vents and thus the functionality.

 

Isn't there a visible sensor/camera visible from the outside looking in?

 

Not that I can see. There is a sensor on the lower black-out portion of the glass centered on the mirror assembly. That makes more sense for automatic headlights.

 

Not sure what's in the black box, but I have my Radar Detector mounted directly below it with a suction cup mount on the windshield. The actual body of the radar detector extends directly out from under that box. Although I haven't sealed those vents, they are definitely slightly obstructed by the detector and I have experienced no troubles.

Here is a snip from a photo from an outside view that I took from a previous post.

 

I'd expect some sort of sensor around there for the automatic wipers.