Headlight Sensor?

Above the rear-view mirror and adhered to the inside of the windshield is a sensor who's purpose I cannot identify. Behind it, a dotted pattern has been applied to that area of the windshield as if to shield the sensor from daylight.

A while ago, the sensor fell off it's adhesive mount and was dangling, so I tucked it in above my sun visor. The next morning I went into the garage and noticed that the parking lights were flashing at a very slow rate. I finally located that sensor as the source of the problem. When I pressed a button on top of it, a small blue LED light came out of the sensor and then the flashing stopped.

I have since re-mounted the sensor in it's proper place, but as an old car guy, I'm just curious about just what the heck that sensor is for.

I'm including a couple of pics.