I have met a dog like that before. She would sit and stare at ANY lights. The moon in the sky during the day. The lights in a room. Car headlights. Strips of sunlight. I believe it was mainly to do with the fact that when she was younger her owners would play with a laser pointer with her. Dogs can get OBSESSED with catching the light because they never can ... if that makes sense.
So perhaps if you see this dog again maybe ask the owners about laser pointers.
"In order to really enjoy a dog, one doesn't merely try to train him to be semihuman. The point of it is to open oneself to the possibility of becoming partly a dog." - Edward Hoagland